LIBRARY 


UNIVERSITY  OF  CALIFORNIA. 

Deceived        (&&£  ,iBgOt 

^Accessions  No.fov0/f.  Class  No. 


»< 
§? 


ELECTRICAL 


ENGINEERING  LEAFLETS 


BY 


PROFESSOR  E.  j.  HOUSTON,  PH.  D. 

\( 

AND 

PROFESSOR  A.  E.  KENNELLY,  F.R.A.S. 


INTERMEDIATE  GRADE 


1895 

THE   ELECTRICAL   ENGINEER 
NEW  YORK 


Engineering 
Library 


'"FHE  Electrical  Engineering  Leaflets  have  been  pre- 
pared for  the  purpose  of  presenting,  concisely 
but  accurately,  some  of  the  fundamental  principles  of 
electrical  science,  as  employed  in  engineering  practice. 
They  have  been  arranged  under  three  grades ;  namely, 
the  Elementary,  the  Intermediate,  and  the  Advanced. 

The  Elementary  Grade  is  intended  for  those  electrical 
artisans,  linemen,  motormkh,  central  station  workmen,  or 
electrical  mechanics  generally,  who  may  not  have  advanced 
sufficiently  far  in  their  studies  to  warrant  their  undertak- 
ing the  other  grades.  Here  the  mathematical  treatment 
is  limited  to  arithmetic,  and  the  principles  are  illustrated 
by  examples  taken  from  actual  practice. 

The  Intermediate  Grade  is  intended  for  students  of 
electricity  in  high  schools  and  colleges.  In  this  grade  a 
certain  knowledge  of  the  subjects  of  electricity  and  physics 
generally  is  assumed,  and  a  fuller  mathematical  treat- 
ment is  adopted.  These  leaflets,  moreover,  contain  such 
information  concerning  the  science  of  electricity,  as  should 
be  acquired  by  those  desiring  general  mental  culture. 

The  Advanced  Grade  is  designed  for  students  taking 
special  courses  in  electrical  engineering  in -colleges  or 
universities.  Here  the  treatment  is  more  condensed  and 
mathematical  than  in  the  other  grades. 

Although  the  three  grades  have  been  especially  pre- 


IV 


pared  for  the  particular  classes  of  students  referred  to, 
yet  it  is  believed  that  they  will  all  prove  of  value  to  the 
general  reading  public,  as  offering  a  ready  means  for  ac- 
quiring that  knowledge,  which  the  present  extended  use 
and  rapidly  increasing  commercial  employment  of  elec- 
tricity necessitates. 

Laboratory  of  Houston  &  Kennelly, 
Philadelphia,  March,  1895. 


CONTENTS. 


GRADE. 


No.  1. 

ELECTRICAL  EFFECTS  

1 

"     2. 

ELECTROMOTIVE  FORCE  

9 

"     3. 

ELECTRIC  RESISTANCE  

17 

..      i 

ELECTRIC  RESISTANCE  

25 

"     5. 

ELECTRIC  RESISTANCE  

33 

"     6. 

ELECTRIC  CURRENT  

41 

"     T. 

OHM'S  LAW  

..       49 

"     8. 

ELECTRIC  CIRCUITS  

..        57 

"     9. 

THE  VOLTAIC  CELL  

..        65 

"  10. 

THE  VOLTAIC  CELL   

73 

"  11. 

THE  VOLTAIC  CELL  

81 

"  12. 

MAGNETOMOTIVE  FORCE  

89 

"  13. 

MAGNETIC  RELUCTANCE  ....    

..       97 

"  14. 

MAGNETIC  FLUX  

..     105 

"  15. 

ELECTROMAGNETS   

.  .     113 

"  16. 

INDUCED  E.  M.  F  

.     121 

"  17. 

THE  DYNAMO  

,  .  .     129 

"  18. 

THE  DYNAMO  

..     137 

"  19. 

THE  DYNAMO  

..     145 

"  20. 

THE  REGULATION  OF  THE  DYNAMO  

153 

PAGE. 

No.  21.     ELECTRODYNAMICS 161 

"     22.     THE  ELECTRIC  MOTOR,  (CONTINUOUS  CUR- 
RENT TYPE) 169 

"     23.     THE  ELECTRIC  MOTOR,  (CONTINUOUS  CUR- 
RENT TYPE) ITT 

"     24.     THE  ELECTRIC  MOTOR,  (CONTINUOUS  CUR- 
RENT TYPE) 184 

"     25.     ELECTRIC  HEATING 193 

"     26.     INCANDESCENT  LIGHTING 201 

"     2T.     INCANDESCENT  LIGHTING 209 

"     28.     ARC  LIGHTING 21T 

"     29.     ARC  LIGHTING 225 

"     30.     ALTERNATING  CURRENTS 233 

"     31.     ALTERNATING  CURRENTS 241 

"     32.     ALTERNATING  CURRENTS 249 

"     33.     ALTERNATORS 25T 

"     34.     ALTERNATORS 265 

"     35.     ALTERNATING  CURRENT  TRANSFORMERS.  .  2T3 


[Copyright,  1894,  by  THE  ELECTRICAL  ENGINEER.] 
WEEKLY. 

No    1  JIT-NT?  1fi    1804  Price,    -    10  Cents. 

Lb>  1  Subscription,  $3.00. 

Electrical    Engineering   Leaflets, 


Prof.  E.  J.  Houston,  Ph. 

A.  «.  «.„„.','•,.,.,,.  A. 


INTERMEDIATE     GRA 

ELECTRICAL 


1.  The  friction  of  a  glass  rod  against  silk  causes 
both  the  glass  and  the  silk  to  acquire  a  property 

they  did  not  previously  possess,  of  attracting  light 
bodies ;  *.  0.,  shreds  of  paper  or  cotton,  in  their  vicinity. 
This  is  an  electrical  effect.  In  addition  to  this,  when 
the  glass  is  vigorously  rubbed,  crackling  sounds  are 
heard,  and,  in  a  dark  room,  faint  gleams  of  bluish  light 
accompany  the  sound.  Moreover,  if  the  rod  while 
vigorously  excited,  be  held  near  the  face,  a  peculiar 
sensation  is  felt,  like  that  caused  by  the  passage  of 
cobwebs  over  the  face. 

Both  glass  and  silk  after  being  rubbed  together,  are 
said  to  have  acquired  an  electric  cha/rge. 

2.  The  exact  nature  of  the  process  whereby  the 
rubbing  together  of  two  substances  produces  an 

electric  charge  is  not  known,  nor  is  the  exact  nature  of 
the  charge  itself  understood. 


Published  by 

THE  ELECTRICAL  ENGINEER, 
203  Broadway,  New  York,  N.  Y. 


It  is  known,  however,  that  tlie  presence  of  an  elec- 
trical charge  on  any  body  or  bodies  is  always  accompanied 
by  a  strained  condition  in  the  surrounding  space ;  but 
whether  this  strained  condition  is  the  cause  of  the 
electrical  charge  or  charges,  or  whether  it  is  their  effect 
is  as  yet  unknown. 

All  space  is  believed  to  be  filled  with  an  extremely 
tenuous,  elastic  medium,  called  the  ether.  The  ether  not 
only  pervades  all  free  space,  but  even  exists  in  the  inter- 
spaces between  the  ultimate  particles  of  all  solid  bodies,  so 
that  it  may  be  said  in  this  sense  to  permeate  all  matter. 
Light  and  radiant  heat  are  particular  forms  of  wave- 
motion-disturbance  in  the  ether;  and  it  is  generally 
believed  that  the  force  of  gravitation  is  transmitted 
through  this  medium. 

3.  Although  the  exact  manner  in  which  the  rub- 
bing together  of  two  bodies  produces  the  strained 

condition  in  the  neighboring  ether  is  not  known,  yet  it 
is  undoubtedly  due  to  the  contact  of  dissimilar  substances. 
When  any  two  dissimilar  substances  are  brought  into 
contact,  even  without  friction,  an  electrical  charge  is 
produced  at  their  contact  surfaces,  varying  in  amount  with 
the  nature  of  the  substances,  as  well  as  with  the  character 
of  their  surfaces ;  i.  e.,  with  the  degree  of  surface  dis- 
similarity, so  to  speak. 

4.  The  charge  which  accompanies  the  contact  of 
two  dissimilar    substances  cannot  be  augmented 

by  continuing  the  friction  between  them  if  both  sub- 
stances are  conductors,  but  it  may  be  very  greatly  aug- 
mented by  continued  successive  surface  contact  or 
friction,  if  one  or  both  substances  are  non-conductors. 


3 


5.  In  a  lightning  flash,  "which  Franklin  proved  by 
his  classic  experiment  with  the  kite  in  1752,  to  be 
a  very  powerful  electric  spark,  the  crackling  sounds  ob- 
served in  the  experiment  with  the  glass  rod,  are  aug- 
mented to  the  intensity  of  thunder.  Lightning  discharges, 
as  is  well  known,  may  fuse  metal  work,  and  rend  or  tear 
masonry. 

G.  The  discharge  of  a  charged  body  by  any  means, 
as  by  a  spark,  produces  momentarily  what  is 
called  an  electric  current ;  and,  indeed,  the  establishment 
of  a  charge  is  also  attended  by  a  current.  In  nearly  all 
such  cases  the  current  is  of  but  momentary  character.  A 
number  of  successive,  momentary  discharges  following 
one  another  with  sufficient  rapidity  produces  an  approxi- 
mate steady  electric  current. 

7.  The  dynamo  electric  machine  is  a  ready  source 
of  powerful  electric  currents.     The   passage  of 

powerful  currents  through  conductors  is  attended  by 
heating  effects.  After  a  dynamo  has  been  generating 
current  for  some  time,  its  coils  of  wire  become  sensibly 
warmed.  "When  passed  through  a  metallic  conductor  an 
electric  current  may  even  melt  or  fuse  the  conductor 
if  the  area  or  cross-section  in  the  latter  is  too  small. 

8.  In  the  incandescent  lamp  the  passage  of  an  elec- 
tric current  through  a  carbon  thread  or  filament, 

raises  it  to  a  high  degree  of  incandescence.  The  fila- 
ment is  enclosed  by  a  glass  chamber,  from  which  all 
the  oxygen  has  been  exhausted,  and  care  is  taken  to 
prevent  the  current  strength  from  becoming  sufficiently 
strong  to  fuse  or  volatilize  the  filament. 

"When  a  powerful  electric  current  is  sent  through  two 


carbon  rods  which  are  first  in  contact,  and  are  then 
gradually  separated  by  about  one-eighth  of  an  inch,  a 
powerful  luminous  discharge  called  the  voltaic  arc  passes 
between  the  carbon  points. 

9.  The  passage  of  an  electric  current  through  a 
conductor  not  only  produces  heat  in  the  conductor, 
but  also  invariably  produces  magnetic  effects  which  are 
readily  observed  under  certain  circumstances.  For  in- 
stance, the  passage  of  a  powerful  electric  current  through 
the  wire  coils  on  the  frame  of  a  dynamo  machine,  pro- 
duces powerful  magnetic  effects,  and  a  bar  of  iron 
brought  near  to  these  magnets  will  be  powerfully  mag- 
netized and  attracted. 

10.  The  electric  current  also  possesses  the  property 
of  decomposing  chemical  solutions  through  which 

it  passes ;  for  example,  if  an  electric  current  be  led,  under 
suitable  conditions,  through  a  solution  of  copper  sulphate 
it  will  decompose  the  salt  in  the  solution,  and  deposit 
metallic  copper  in  a  coherent  and  adherent  layer  upon 
any  conducting  surface  suitably  connected  with  the  lead- 
ing-in  wires.  This  decomposition  is  called  electrolysis. 

11.  Electric  currents,  therefore,  produce  a  variety  of 
effects  which  may  be  grouped  as  follows  : — 

(I)  Luminous,  as  in  sparks  or  in  electric  lamp. 
(#)  Thermal,  or  heating,  as  in  fusion  of  wire. 


Electrical 
Effects. 


(3)  Mechanical,  as  in  the  disruptive  effects 

of  lightning  discharge. 

(Jf)  Physiological,  as  in  shock  to  human  body. 
(5)  Magnetic,  as  in  dynamo  magnet. 
,.  (6)  Electrolytic,  as  in  electroplating. 


12.  It    is    well    known    that   in   order   to   start  a 
body  in   motion  or   to  change  the  direction  or 

velocity  of  its  motion,  force  must  be  applied.  Thus  a 
train  of  cars  at  rest  requires  the  action  of  the  steam  en- 
gine to  set  it  in  motion,  and  when  in  motion,  the  action 
of  the  brakes  to  bring  it  to  a  standstill. 

A  baseball  requires  a  certain  force  to  project  it  with  a 
certain  initial  velocity  and  can  only  be  stopped  by  the 
application  of  opposing  forces. 

13.  "Whenever  force  acts  through  a  distance,  it  is  said 
to  do  work,  and,  in  the  cases  just  considered,  the 

motion  of  the  body  tinder  the  force  is  an  evidence  of  the 
performance  of  work.  The  word  energy  is  employed 
in  the  sense  of  capability  of  doing  work,  or  as  a  store  of 
work,  and  when  work  is  done,  energy  is  expended,  and 
some  store  of  work  has  been  drawn  upon. 

The  performance  of  any  work  whatever,  therefore, 
necessitates  the  expenditure  of  energy. 

14.  "When  a  railroad  train  is  set  in  motion,  steam  has 
to  do  work  by  moving  the  pistons  to-and-f  ro  in  the 

cylinders,  thus  exerting  force  through  a  distance.  The 
steam  derived  its  energy  from  the  burning  of  the  coal 
under  the  boiler,  and  the  coal  in  its  turn  originally 
derived  its  energy  from  the  sun,  through  the  absorption 
of  the  sun's  radiant  energy  by  the  vegetable  matter  from 
which  the  coal  was  formed. 

When  a  baseball  is  set  in  motion,  the  energy  of  the 
moving  ball— its  power  of  overcoming  obstacles — is 
obtained  from  the  muscular  power  of  the  thrower  who 
thus  exerted  muscular  force  through  a  distance.  This 
muscular  energy  was  originally  derived  from  the  food 


6 


assimilated  by  liis  body.  In  its  turn,  the  food  derived 
Us  energy  from  the  sun's  radiant  heat. 

When  a  dynamo  is  generating  an  electric  current,  its 
driving  belt  is  exerting  force  upon  the  rim  of  the  pulley, 
thus  moving  it  through  distance  and  therefore  doing 
work.  It  is  upon  this  store  of  work  that  the  electric 
current  has  to  draw  for  the  accomplishment  of  any  of 
its  above  mentioned  characteristic  effects. 

Fig.  1  shows  the  electrical  transmission  of  power  as 
contrasted  with  Pig.  2  which  shows  the  mechanical  trans- 


ELECTRICAL  TRANSMISSION  OF  POWER 


FIG.  I. 

mission  of  power  by  means  of  a  rope.  In  each  case  the 
falling  weight  drives  the  generator,  and  a  motor  lifts  a 
smaller  weight.  The  work  done  by  the  motor  is  less 
than  the  work  expended  on  the  generator  by  an  amount 
equal  to  the  loss  in  transmission. 

15.       It  is   a  well  established  principle  in   science 

that  the  total  amount  of  energy  in  the  universe 

is  constant.     All  natural  phenomena  are  dne  to  a  change 


of  form  in  the  energy  manifested  when  force  acts  on 
matter,  and  throughout  all  these  changes  whatever  energy 
disappears  in  one  form  reappears  in  some  other  form. 

16.       In  every  transformation  some  energy  is  expended 

in   a  direction  iu  which  it   cannot  be  utilized; 

that  is,  in  effects  which  are  not  desired ;   such  diverted 

energy  is  called  wasted  energy,  but  is  only  truly  wasted 

from  an  utilitarian  point  of  view. 

Since  energy,  like  matter,  is  indestructible,  it  is  evi- 
dent that  the  total  work  done,  or  the  energy  which  appears 


FIG.  2. 

in  the  performance  of  any  machine,  must  always  be  ex- 
actly equal  to  the  work  expended  in  driving  it,  the 
intake  /  but  the  amount  of  energy  turned  to  useful  ac- 
count, the  output,  is  always  less  than  the  intake, 

17.      The  ratio  between  the  output  and  the  intake,  that 

is,  the  output  divided  by  the  intake,  is  called  the 

efficiency  of  the  machine,  and  is  always  less  than  unity ; 

^   Of  TH« 


s 


for  example,  the  efficiency  of  very  large,  well  constructed 
dynamos  is  about  0.95. 

18.       From  what  has  been  said,  it  will  be  recognized 
that  the  electrical  machine  forms  no  exception  to 
the  universal  rule  that  to  produce  any  effect  a  correspond- 
ing expenditure  of  work  in  some  form  is  necessary. 

SYLLABUS. 

An  electric  charge  is  accompanied  by  a  strained  con- 
dition in  the  ether  surrounding  the  charged  body. 

The  electric  charge  produced  by  friction  has  its  origin 
in  the  contact  of  dissimilar  molecules. 

The  passage  of  an  electric  current  through  a  conductor 
is  always  attended  by  the  production  of  a  magnetic  field. 

When  an  electric  current  is  passed  through  a  solution 
of  copper  sulphate  under  suitable  conditions,  a  decom- 
position called  electrolysis  is  effected. 

Work  is  never  done  or  energy  expended  unless  force 
acts  through  a  distance.  Energy  is  never  created  ;  there- 
fore, when  work  is  done  some  previously  existing  store 
of  energy  is  drawn  upon.  The  total  store  or  quantity  of 
energy  in  the  universe  is  constant. 

Every  electrical  effect  is  due  to  energy  expended,  and 
the  amount  of  such  work  can  generally  be  calculated. 

The  total  work  done  by  any  machine  must  always 
exactly  equal  in  amount  the  work  expended  in  driving 
it,  but  the  useful  work  done  by  the  machine  is  always  less 
than  the  work  expended  in  driving  it.  The  ratio  of  the 
output  of  any  machine  to  the  intake  is  called  the  effici- 
ency and  is  always  less  than  unity. 

Laboratory  of  Houston  and  Kennelly, 
Philadelphia. 


[Copyright,  1894,  by  THE  ELECTRICAL  ENGINEER.! 
WEEKLY. 


No.  2.  JUNE  23,  1894. 

Electrical    Engineering   Leaflets, 


Prof.  E.  J.  Houston,  Ph.  D. 

AND 

A.  E.  Kennelly,  F.  R.  A.  S. 


INTERMEDIATE     GRADE. 

ELECTROMOTIVE  KORCE 


19.  Electromotive  force  is  the  name  given  to   the 
unknown  force  or  cause  whiclj  produces,  or  tends 

to  produce,  an  electric  current. 

Whenever  an  electric  current  flows  in  a  circuit  such 
current  is  due  to  an  electromotive  force  (abbreviated 
E.  M.  F.)  acting  on  that  circuit. 

Just  as  a  mechanical  force  acting  on  a  body  produces 
or  tends  to  produce  motion  in  that  body,  so  an  electro- 
motive force  acting  on  a  circuit,  produces  or  tends  to 
produce  an  electric  motion ;  i.  e.,  current  in  that  circuit. 

An  E.  M.  force,  like  all  other  forces,  possesses  a  definite 
direction,  and  as  all  forces  tend  to  produce  motion  in 
their  direction,  so  an  E.  M.  F.  tends  to  produce  current  in 
its  direction. 

20.  As   in   mechanics,  two   or   more  forces,  when 
simultaneously  acting  may,  when  opposed,  neu- 
tralize each  other  and  thus  produce  no  motion ;  or,  when 
acting  in  the  same  direction,  may  aid  each  other  and  thus 


Published  by 

THE   ELECTRICAL  ENGINEER, 
203  Broadway,  New  York,  N.  Y. 


10 


produce  increased  motion,  so  two  or  more  E.  M.  forces 
acting  simultaneously  on  the  same  circuit,  when  opposed 
may  neutralize  each  other,  and  thus  produce  no  current ; 
or  when  acting  in  the  same  direction,  may  aid  each  other, 
and  thus  produce  a  stronger  current. 


FIG.  3. 

Thus  in  Fig.  3  the  man  exerting  a  steady  mechanical 
force  moves  a  car  weighing  one  ton  along  a  level  track 
at  a  rate  of  one  mile  an  hour,  and  the  single  voltaic 
cell,  symbolized  as  shown  by  two  lines  of  unequal  length 
and  thickness,  produces  an  E.  M.  r.  which  sends  a  certain 
current  through  the  conducting  circuit. 

"When,  however,  as  in  Fig.  4,  the  two  men  apply 
simultaneously  equal  mechanical  forces  to  the  car  in 
opposite  directions,  a  neutralization  or  balance  is  effected 
and  no  motion  is  produced.  So  the  two  voltaic  cells 


I 


£Uc.£nyineer 

FIG.  4. 


connected  in  opposition  in  the  same  circuit  neutralize  or 
balance  each  other  and  no  current  is  produced. 

Again  in  Fig.  5  the  two  equal  mechanical  forces  ap- 
plied simultaneously  in  the  same  direction,  move  a  car 
weighing  two  tons  at  the  rate  of  one  mile  an  hour,  and 


11 


the  two  voltaic  cells  connected  in  series,  so  that  their  E. 
M.  F.'S  aid  each  other,  produce  a  double  E.  M.  F.  that  can 
send  twice  the  current  through  the  circuit. 

An  E.  M.  F.  may,  therefore,  originate  a  current,  may 
increase  the  strength  of  a  current  already  existing,  or 
may  oppose  and  weaken  or  altogether  neutralize  such 
current. 

21.  E.  M.  F.  is  measured  in  units  named  volts.  Large 
E.  M.  F.'S  are  sometimes  expressed  in  kilovolts  and 
small  E.  M.  F.'S  in  millivolts  or  microvolts,  (i.  <?.,  thou- 
sanths  and  millionths  of  a  volt).  Thus  the  E.  M.  F.  pro- 
duced by  a  frictional  or  influence  machine  which  will 
cause  an  electric  spark  discharge  over  an  air  space  of 


FIG.  5. 

one  inch  between  brass  knobs  might  be  75,000  volts, 
or  75  kilovolts.  The  E.  M.  F.  of  a  gravity  or  bluestone 
cell  such  as  is  frequently  used  in  telegraphy  is  about  1.08 
volts. 

The  usual  standard  of  E.  M.  F.  for  testing  and  com- 
parative purposes  is  a  special  form  of  voltaic  cell  called 
the  Clark  cell.  It  is  composed  of  pure  chemicals  and 
made  with  great  care.  It  is  accepted  as  having  an  E.  M.  F. 
of  1.434  volt  at  15°C. 

22.       The  joule  is  the  international  unit  of  work  and 

is  equal  to  0.738  foot-pounds  at  the  latitude  of 

Washington;  or,  in  other  words,  one  joule  of  work  will 


raise  a  pound  of  matter  at  the  latitude  of  Washington 
through  the  distance  of  0.7381  foot. 

The  watt  is  the  international  unit  of  activity  or  power, 
or  of  the  rate  of  working  per  second  of  time,  and  is  an 
activity  of  one  joule  per  second.  That  is  to  say,  a  force 
which  will  raise  one  pound  at  the  latitude  of  Washington 
through  the  distance  of  0.738  foot  in  a  second,  expends 
work  at  the  rate  of  one  watt,  or  one  joule  per  second. 

The  prefixes  of  J&ilo-  and  mega-  are  employed  for  the 
multiples  of  1,000  and  1,000,000  respectively,  and  are 
often  used  as  convenient  abbreviations.  Thus  a  kilowatt 
is  1,000  watts,  and  represents  738  foot-pounds  expended 
per  second  at  Washington.  The  "horse-power"  is  a 
unit  of  activity  introduced  by  James  Watt,  and  is  in 
common  engineering  use.  It  represents  an  activity  of 
550  foot-pounds  per  second  at  Greenwich  or  746  watts; 
so  that, 

1  H.  p.   =  0.746  kilowatt, 
or  1  K.  w.     =  1.34   H.  P. 

23.  It  is  necessary  to  carefully  distinguish  between 
work  or  energy  (joules)  and  activity  or  power 
(watts).  Work  is  an  expenditure  of  energy,  and  is  equal 
to  the  product  of  a  force  and  the  distance  through  which 
that  force  acts.  Activity  is  the  rate  of  expending  energy 
or  doing  work,  and  is  found,  or  at  least  averaged,  by 
dividing  the  work  done  by  the  time  occupied  in  doing  it. 
The  same  amount  of  work  is  done  when  a  weight  of  one 
pound  is  raised  through  one  foot,  whether  it  be  raised  in 
minute  or  in  one  second,  but  the  rate  at  which  the  work  one 
is  done,  or  energy  expended,  is  sixty  times  greater  in  the 
latter  case. 


13 


24.  E.  M.  F.'S  are  produced  by  devices  called  electric 
sources.    Electric  sources  are  of  various  kinds  and 

may  be  conveniently  grouped  as  follows. 

Dynamo  electric  machines.  (Producing  E.  M.  F.  from 
1  to  7000  volts  or  more.) 

Yoltaic  cells.  (Producing  singly  E.  M.  F.  of  0.5  to  2.5 
volts.) 

Thermo-electric  couples.  (Producing  an  E.  M.  F.  of  a 
few  millivolts.) 

Frictional  electric  machines.  (Producing  an  E.  M.  F. 
of  many  kilo  volts.) 

The  terminals  of  an  electric  source,  i.  e.,  the  points 
from  which  the  current  is  assumed- to  leave  the  source, 
and  again  enter  it  after  having  passed  through  the  cir- 
cuit, are  termed  its  poles  or  terminals ;  the  pole  from 
which  the  current  is  conventionally  assumed  to  leave 
the  source  being  called  the  positive  pole^  and  that  at 
which  it  again  enters  the  source  after  having  passed 
through  the  circuit  being  called  the  negative  pole. 

It  has  been  pointed  out  (Par.  14)  that  unless  a  mechan- 
ical force  moves  through  a  distance,  it  does  no  work.  In 
the  same  way,  an  E.  M.  F.  does  no  work  unless  it  produces  a 
current.  But  when  an  E.  M.  F.  produces  a  current  in  a 
conducting  circuit  it  does  work,  and  the  energy  neces- 
sary to  do  this  work  is  supplied  by  the  electric  source. 

25.  From  an  engineering  standpoint  the  most  im- 
portant  electric   source  is   the   dynamo    electric 

machine. 

The  first  dynamo  electric  machine,  a  mere  toy  in  point 
of  dimensions,  was  invented  and  constructed  by  Faraday, 
in  1831.  At  the  present  time  dynamos  are  made  up  to 
3,750  kilowatts  capacity. 


The  voltaic  cell  is  the  next  electric  source  in  order  of 
practical  importance.  The  output  of  a  cell  is,  however, 
very  small  by  comparison  with  an  ordinary  dynamo,  and 
rarely  exceeds  15  watts.  A  number  of  cells  connected 
together  so  as  to  form  a  single  source  or  battery ,  can,  of 
course,  increase  the  output  proportionally.  Batteries, 
however,  are  never  used  for  the  production  of  any  con- 
siderable amount  of  electric  power. 

26.  Electromotive  forces  differ  not  only  in  magni- 
tude, but  in  variability,  or  time-rate-of-change. 
Thus  arise  two  general  divisions  of  E.  M.  F.,  viz.,  the  con- 
tinuous and  the  alte> nat'ng.  An  E.  M.  r.  that  has  always 
the  same  direction  is  said  to  be  continuous.  An  E.  M.  F.  that 
alternately  and  periodically  reverses  its  direction,  is  said 
to  be  alternating.  As  a  continuous  E.  M.  F.  produces,  or 
tends  to  produce,  a  continuous  current,  so  an  alternating 
E.  M.  F.  produces,  or  tends  to  produce,  an  alternating  or 
oscillating  current. 

Continuous  E.  M.  F.'S  are  further  divided  into  steady 
and  pulsating.  Steady  E.  M.  F.'S  are,  for  Ibrief  periods 
at  least,  practically  produced  by  voltaic  batteries  and 
thermopiles.  Continuous  current  dynamos  (so-called), 
always  produce  in  reality,  fluctuating  or  pulsating  E.  M. 
F.'S  although  the  fluctuations  may  in  some  dynamos  be 
so  slight  and  rapid  as  to  escape  notice. 

Alternating  E.  M.  F.'S  are  either  symmetr  'cat  or  dis- 
symmetrical^ according  as  the  positive  and  negative 
waves,  allowing  for  changes  of  direction,  are  or  are  not 
similar  and  equal.  Alternating  current  dynamos  (alter- 
nators] usually  produce  symmetrical  E.  M.  F.'S,  while  a 
Ruhmkorff  coil  with  a  vibrating  spring,  operated  by  a  con- 
tinuous E.  M.  F.,  produces  a  dissymmetrical  alternating  E.M.F. 


15 


The  K.  M.  F.  at  breaking  contact  being  much  greater  than 
the  oppositely  directed  E.  M.  F.  at  making. 

27.  If  as  in  (Fig.  0.)  the  water  in  the  vessel  A,  is  in 
communication  with  the  open  vertical  tubes  a,  &, 
c,  d,  e,f9  (7,  then  wlien  the  outlet  tube  B  is  closed,  the 
level  at  which  the  water  stands  will  be  the  same  in  all 
the  tubes.  But  when  the  outlet  is  open,  the  level  will 
be  highest  in  the  tube  nearest  to  the  reservoir,  and 
lowest  in  the  tube  nearest  to  the  outlet,  the  level  in  the 
intermediate  tubes  being  found  along  the  inclined  dotted 


d' 


^6' 


£ 


I  | 

£ L 

+ci        ft 


f 

.  Engineer 


FIG.  6. — Hydraulic  and  Potential  Gradients, 
line  a' ',  £>',  <?7,  d ' ,  er,f'9  which  line  may  be  called  the  hy- 
draulic gradient.  The  force  which  causes  the  water  to 
flow  through  the  tube  CB,  which  may  be  called  the  water- 
motive-force,  is  that  due  to  the  difference  of  level  be- 
tween A  and  B.  Of  this  force,  that  fraction  which  causes 
the  water  to  flow  between  any  two  points  of  the  tube 
CB  as  for  example  between  .a  and  5,  is  that  due  to  the 
difference  of  level  between  of  and  J'. 

Similarly,  the  E.  M.  F.  produced  by  the  electric  source 
or  dynamo  A,  which  has  its  positive  pole  connected  to  c, 


16 


and  its  negative  pole  connected  to  the  ground  produces 
a  fall  of  potential  or  electric  pressure  along  the  uniform 
conducting  wire  CB,  which  is  connected  to  the  ground  at 
B.  The  gradient  of  electric  potential  being  represented 
by  the  dotted  line  a',  I ',.«' '>  <*'>  «',/'>  an^  the  E.  M.  F. 
which  drives  the  current  through  any  portion  of  the  con- 
ductor such  as  a  c  may  be  attributed  to  the  difference  of 
potential  between  a'  and  c' ,  as  represented  by  the  differ- 
ence of  length  between  the  lines  a  a'  and  c  c'  measured 
in  volts. 

As  water  flows  from  a  higher  to  a  lower  level,  so  the 
electric  current  is  assumed  to  flow  from  a  higher  to  a 
lower  potential ;  and  as  differences  of  water  level  con- 
stitute what  has  been  called  water-motive  force,  so 
difference  of  electric  potential  constitutes  electromotive 
force.  The  sum  of  all  the  differences  of  potential  (ab- 
breviated P.D.'S)  in  a  circuit  is  equal  to  the  total  E.  M.  F.  in 
that  circuit. 

SYLLABUS. 

An  E.  M.  F.  has  a  definite  direction  and  tends  to  pro- 
duce an  electric  current  in  that  direction. 

E.  M.  F.  is  measured  in  practical  units,  called  volts, 
also  in  micro-,  milli-,  and  kilovolts. 

The  international  unit  of  work  is  called  the  joule. 

The  international  unit  of  activity ;  i.  <?.,  the  joule-per 
second,  is  called  the  watt. 

An  E.  M.  F.  does  no  work  unless  it  produces  a  current. 

E.  M.  F'S  differ  in  both  magnitude  and  in  variability. 
They  are,  therefore,  continuous  and  alternating.  Con- 
tinuous E.  M.  F'S  may  be  steady  or  pulsating. 

Laboratory  of  Houston  &  Kennelly, 
Philadelphia. 


[Copyright,  1894,  by  THE  ELECTRICAL  ENGINEER."] 
WEEKLY. 


TS'o    3  JrvK  SO    1  S(U  IVicr,          llMYnfs. 

*U>  -1  Subscription,  $3.00. 

Electrical    Engineering   Leaflets, 


Prof.  E.  J.  Houston,  Ph.  D. 

AND 

A.  E.  Kennelly,  F.  R.  A.  S. 


INTERMEDIATE     GRADE 

ELECTRIC 


28.  Resistance  is  that  property  of  an  electric  con- 
ductor or  circuit,  in  virtue  of  which  an  electric 

current  or  flow  is  limited,  under  any  given  E.  M.  F.,  to  a 
certain  value. 

The  resistance  of  a  water-pipe  to  the  passage  of  water 
through  it,  increases  directly  with  the  length  of  the  pipe, 
and  diminishes  with  the  cross  sectional  area  of  the  pipe. 
It  also  varies  with  the  nature  of  the  material  from  which 
the  pipe  is  made,  and  the  smoothness  of  its  inner  sur- 
face. So  too,  the  resistance  of  a  circuit  or  conductor, 
varies  directly  with  the  length  of  the  conductor,  and  in- 
versely as  its  cross-sectional  area.  It  also  varies  with 
the  nature  and  physical  conditions  of  the  materials  of 
which  the  conductor  is  composed. 

29.  Electric  resistances  are  compared  with  one  an- 
other by  reference  to  certain  practical  electrical 

units.     The  unit  of  resistance  is  called  the  International 
ohm,  and  is  the  resistance  offered  by  a  pure  chemical 

Published  by 

THE   ELECTRICAL  ENGINEER, 
203  Broadway,  New  York,  N.  Y. 

[Entered  as  second-class  matter  at  the  New  York,  N.  Y.,  Post/Jfiu^Junexj,    :rH* 


UKIVBRSIT7 


18 


material  of  definite  dimensions  under  fixed  physical  con- 
ditions. 

The  value  of  the  ohm  is  most  conveniently  taken  as 
the  resistance  of  a  column  of  pure  mercury,  one  square 
millimetre  in  area  of  cross-section  and  106.3  centimetres  in 
length,  at  the  temperature  of  melting  ice.  (Zero  centi- 
grade or  32°  F.)  Its  value,  however,  can  be  defined  in 
terms  of  any  conducting  material,  such  as,  approximately, 
one  foot  of  No.  42  A.  w.  G.  wire  of  pure  copper  ;  and 
such  a  wire  might  be  more  conveniently  maintained  as  a 
material  standard  than  .a  column  of  liquid  mercury;  but 
although  mechanically  advantageous,  such  a  standard 
would  possess  the  inconvenience  that  no  two  samples  of 
copper  wire  could  be  obtained  of  exactly  the  same  degree 
of  purity  and  in  the  same  physical  condition,  while  mer- 
cury, by  redistillation,  can  be  readily  obtained  chemically 
pure,  and  in  the  same  physical  condition. 

The  fundamental  unit  of  electric  -resistance,  in  the 
International  c.  a.  s.  system,  is  that  resistance  in  which 
the  unit  of  electric  current  will  do  work  to  the  extent 
of  one  erg  (one  dyne-centimetre)  in  one  second.  This 
resistance  is,  however,  so  extremely  small  that  it  would 
be  impracticable  to  employ  it  directly,  and  another  unit, 
the  ohm,  was  selected  as  the  practical  unit  of  electric 
resistance.  The  ohm  is  equal  to  one  billion  (1,000,000,< )( )( ) 
or  109),  of  the  fundamental  c.  G.  s.  units  of  resistance. 

30.  Conductors  are  generally  made  in  the  form  of 
wires.  A  circular  cross-section  is  the  commonest, 
though  a  rectangular  section  is  sometimes  employed  in 
dynamo  armature-winding  for  economy  of  space. 

Doubling  the  length  of  a  wire  doubles  its  resistance. 
Similarly,  halving  the  length  of  a  wire  halves  its  re- 


19 


sistance ;  doubling  the  area  of  cross-section  also  halves 
its  resistance.  Consequently,  if  a  wire  of  given  length 
be  cut  into  two  equal  parts,  and  the  two  lengths  be  laid 
side  by  side,  and  so  connected  with  the  circuit  that  the 
current  passes  through  them  in  parallel,  their  joint  re- 
sistance, i.e.,  the  resistance  of  the  two  in  combination, 
will  be  one-fourth  of  the  original  resistance  of  the  wire. 
We  have  seen  that  the  resistance  of  a  conductor  varies 
with  the  nature  of  the  material  of  which  it  is  composed. 
As  a  rule,  metals  offer  a  comparatively  low  resistance  to 
the  passage  of  an  electric  current,  and  are,  therefore, 
called  electric  conductors,  while  hard  rubber,  glass, 
gutta-percha,  air,  etc.,  offer  a  comparatively  high  resist- 
ance to  the  passage  of  an  electric  current  and  are,  there- 
fore, called  non-conductors  or  insulators. 

31.       For  large  multiples  or  submultiples  of  an  ohm, 
or  of  any  other  unit,   various  prefixes   are  em- 
ployed ;  as  for  example,  the  following  multiples, 

deka . .  .ten  times 10. .  10 

hecto. . .  one  hundred  times 100 . .  102 

kilo. . .  .one  thousand  times 1,000.  .  103 

mega. . .  one  million  times 1,000,000 .  106 

bega.. .  .one  billion  times 1,000,000,000.  .109. 

trega  .. one  trillion  times ..      ...  1,000,00ft, 000, 000.  .1012 
quega  .  .one  quadrillion  times.. .  .1,000,000,000,000,100. .  101 5 

and  the  following  submultiples  or  decimals, 

deci...  .or  one  tenth 1  -*-  10 1C'1 

centi or  one  hundredth.  ..!-*-  100 10~8 

milli . . ..or  one  thousandth  . .  1  •*•  1,000 10- :l 

micro.  ..or  one  millionth. . .  .1  •*•  1,000,000 10-6 

bicro... or  one  billionth 1  -*•  1,000,000,000 10~9 

tricro. .  .or  one  trillionth.  ...In-  1,000,000,000,000. . .  .UM  * 


One  c.  G.  s.  unit  of  resistance  is.  therefore,  a  bicrohm. 

One  millionth  of  an  ohm  is  a  microhm. 

One  million  ohms  is  a  megohm ;  a  billion  ohms,  a 
begohm;  a  trillion  ohms  is  a  tregohm;  and  a  quadrillion 
ohrns,  a  quegohm. 

32.  For  convenience  in  comparing  the  resistances  of 
different  kinds  of  material,  the  standard  of  com- 
parison is  taken  as  the  resistance  of  unit  length  and  unit 
area  of  cross-section;  namely,  the  resistance  which  would  be 
offered  by  a  cube  of  the  material,  one  centimetre  in  length 
of  edge,  between  opposite  faces.  The  particular  resist- 
ance of  a  body  referred  to  unit  dimensions  in  this  way  is 
called  its  specific  resistance  or  resistivity.  For  example, 
iron  has,  at  ordinary  temperatures,  a  resistance  about  six 
and  a  half  times  that  of  copper.  Thus  copper  of  stand- 
ard purity  (Matthiessen's  standard)  has  a  resistivity  of 
1594  c.  G.  s.  units,  at  the  melting  point  of  ice,  or  1.594 
microhms,  while  iron  at  the  same  temperature,  has  a 
resistivity  of  9.687  microhms. 

A  study  of  the  following  table  will  show  that,  as  a 
rule,  apart  from  the  metals,  solids  possess  the  highest  re- 
sistivities, or  are  the  poorest  conductors.  Of  liquid  sub- 
stances, oils  possess  the  highest  resistivity.  Water,  as 
measured  by  Kohlrausch,  has  a  resistivity  of  3.75  meg- 
ohms. As,  however,  minute  traces  of  impurity  enor- 
mously diminish  this  resistivity,  it  is  generally  believed 
that  absolutely  pure  water  would  be  almost  a  perfect  in- 
sulator. Water  containing  30  per  cent,  of  its  weight  of 
nitric  acid  has  a  resistivity  of  1.29  ohms. 

The  following  is  a  table  of  resistivities  in  International 
ohms. 


21 


Substance. 

Tempera- 
ture. 

Resistivity. 

Tem- 
perature 
Co- 
efficient 

Authority. 

Silver,  annealed... 
Silver,  hard  drawn 
Copper,     annealed 
(M  a  1  1  h  i  e  ssen's 
standard)     

0°C. 

1.500  microhms 
1.53 

1.594 

0.377 

0.388 

Matthiessen. 

(  'opper,hard  drawn 
Iron,  annealed.  .  .  . 
Nickel,  annealed.  . 
Mercury,  liquid  .  .  . 
(  ierni  an  silver  .... 

1.629 
9.687 
12.420 
94.84 
ab't  20.9 

6'072 
0.044 

Graphite  

Sulphate    of    zinc, 
saturated      solu- 
tio  n 

« 
10 

from  0.0024  to 
0.042  ohms 

33  6  ohms  .  . 

\  about 
f    0.5 

j-  Everett. 

Ewing  & 
Macgregor. 

Common  salt,  solu- 
tion of  minimum 
resistivity 

10° 

47     "     

Kohlrausch 
&Nippoldt. 

Pure  water      •  ' 

••  { 

20° 

abont  3.75  meg- 
ohms 

84  tregohms  .. 

,:::: 

Kohlrausch. 
Ayrton  & 

Gutta-percha  

Hard  rubber  
Paraffin 

24° 

46° 
46° 

449 

28  quegohms  . 
34 

:::: 

Perry. 
Latimer 
Claik. 
Ayrton  & 
Perry. 

Glass,  flint  

0° 

16700      " 

Foussereau  . 

Porcelain  

0° 

540 

i  < 

The  fourth  column  gives  the  temperature  coefficient, 
that  is  the  percentage  increase  in  resistivity  per  degree 
centigrade  increase  in  temperature.  Thus,  copper  in- 
creases 0.388  per  cent,  per  0°  C.,  within  a  range  of  a 
few  degrees  centigrade,  according  to  Matthiessen' s  obser- 
vations, and,  taking  its  resistivity  as  1.594  microhms  at 
0°  C.  at  5°  C.,  its  resistivity  would  be  1.594  (1  -f  5  X 
0.00388)  =  1.594  X  1.0194  =  1.625  microhms. 


22 


From  this  table  of  resistivity  it  is  possible  vto  arrive  at 
the  resistance  of  any  uniform  conductor  whose -resistivity 
is  given.  Thus,  if  the  resistance  of  a  mile  of  Matthies- 
sen's  standard  hard-drawn  copper  wire,  having  a  cross 
section  of  one  square  millimetre  is  required,  for  0°  C., 
we  take  the  resistivity  of  1.629  microhms,  and  since  this 
would  be  the  resistance  of  a  bar  one  centimetre  long 
and  one  square  centimetre  in  cross  section,  the  resistance 
of  a  centimetre  length  of  wire  of  1  square  millimetre 
cross  section  (100  times  less  area)  would  be  100  X  1.629 
=  162.9  microhms,  and  the  resistance  of  a  mile  (160,933 
cms.)  of  this  wire  would  be  162.9  X  160,933  =  26,215,- 
986  microhms  —  26.2  ohms  approximately.  The  pro- 
blem of  finding  the  resistance  of  any  wire  thus  resolves 
itself  into  determining  its  resistivity  at  the  required 
temperature,  its  cross  sectional  area  in  sq.  cms,  and  its 
length  in  cms. 

The  following  table  will  be  useful  in  these  calculations, 
1  inch  =  2.54  cms.        1  sq.  in.  —  6.4516  sq.  cms. 
1  foot  =  30.48  cms. 
1  mile  =  (1760  yds)  =  160,933  cms. 

The  temperature  coefficient  of  carbon  is  negative; 
namely  about  —  0.5  ;  that  is,  a  wire  or  filament  of  car- 
bon; diminishes  about  0.5  per  ^C.  increase,  for  a  small 
range  of  temperature.  At  the  temperature  at  which 
glow  lamps  are  ordinarily  operated,  their  resistance  is 
about  half  what  it  is  when  cold. 

The  resistivity  of  insulating  substances,  diminishes 
like  carbon,  with  temperature,  and  this  in  fact  forms  a 
criterion  as  to  the  class  of  substances  (conductors  or 
insulators)  to  which  a  body  belongs. 


23 


33r.       Condiicta/nce  is  the  inverse  of  resistance,  just  as 
conductivity  is  the  inverse  of  resistivity. 

If,  as  in  Fig.  7,  i'our  incandescent  lamps  be  connected 
to  supply  mains  as  shown,  and  each  lamp  has  when  hot 
a  resistance  of  .100  ohms,  (a  conductance  of  yj-g-  or 
0.01  mho),  the  total  conductance,  since  the  current  is 
conveyed  equally  through  four  different  paths,  will  be 
the  sum  of  the  separate  conductances,  i.  <?.,  0.04  mho,  • 
and  the  effective  or  joint  resistance  of  the  combination 
will  be  -jj-^  =  25  ohms. 

Just  as  the  total  resistance  of  a  number   of  separate 


10  OHMS        -^  MHO 

-AAAAAA/VWWVVV 


FIG.  7.  FIG.  8. 

RESISTANCES  IN  PARALLEL.  RESISTANCES  IN  PARALLEL. 

resistances  in  series  is  the  sum  of  those  resistances,  so  the 
total  conductance  of  a  number  of  separate  conductances 
in  parallel,  is  the  sum  of  those  conductances.  Thus, 
if  three  wires  of  5, 10  and  15  ohms  respectively,  be  con- 
nected in  multiple,  as  shown  in  Fig.  8;  their  respective 
conductances  will  be  -J-  ==  0.2,  TV  —  0.10,  and  -^  = 
0.0067  mho.  The  total  conductance,  therefore,  will  be 
0.3067  mho,  and  the  joint  or  effective  resistance  of  the 
three  wires,  ¥.^T  —  2-727  ohms. 

SYLLABUS. 

Resistance  is  that  property  of  a  conductor  or  circuit 
which  opposes  the  flow  of  electricity  through  it. 


The  resistance  of  a  conductor  varies  with  its  length, 
cross  sectional  area,  and  the  nature  of  its  material. 

Resistances  are  compared  with  each  other  by  reference 
to  a  practical  standard  called  the  International  ohm ; 

The  fundamental  c.  G.  s.  unit  of  resistance  is  one 
bicrohm. 

The  effective  resistance  of  two  or  more  wires  in 
parallel  is  called  their  joint  resistance. 

The  resistance  of  a  body  referred  to  unit  dimensions 
is  called  its  specific  resistance  or  resistivity. 

Laboratory  of  Houston  &  Kennelly. 
*  Philadelphia. 


(.Copyright,  1894,  by  THE  ELECTRICAL  ENGINEER. "] 
WEEKLY. 

~NY>    4  TTTT  v  7    1 8Q4.  Price,     -    10  Cents. 

ULY  7>  *  Subscription,  $3.00. 

Electrical    Engineering   Leaflets, 


Prof .  E.  J.  Houston,  Ph.  D. 

AND 

A.  E.  Kennelly,  F.  R.  A.  S. 


I  NTEF?JYl  EDIATE      GRADE. 

ELECTRIC 


34.  The  conductivity  of  a  material  is  the  inverse  of 
its  resistivity,  so  that  the  greater  the  resistivity, 

the  lower  the  conductivity.  Thus  the  solution  of  nitric 
acid  in  water,  which  gives  the  lowest  resistivity  (1.^9 
ohms),  gives  the  highest  conductivity  T.V-g-  =  0.77519, 
and  any  other  mixture  of  nitric  acid  and  water  would 
conduct  less  perfectly,  that  is,  would  have  a"  lower  con- 
ductivity. Conductivity  is  measured  in  units  called 
mhos,  a  term  derived  from  the  reverse  spelling  of  the 
word  ohm. 

35.  The  resistance  of  two  or  more  conductors  con- 
nected in  series,  that  is,  joined  end  to  end,  is  the 

sum  of  their  separate  resistances.  Thus  three  wires 
which  have  respectively  5,  10  and  15  ohms  resistance, 
have,  when  connected  in  series,  a  total  resistance  of  30 
ohms.  The  total  resistance  in  a  series  circuit  is,  there- 
fore, the  sum  of  all  the  resistances  in  the  different  parts 
of  that  circuit. 


Published  by 

THE  ELECTRICAL  ENGINEER, 
203  Broadway,  New  York,  N.  Y. 

[Entered  as  second-class  matter  at  the  New  York,  N.  Y.,  Post  Office,  June  14,  1894.] 


26 


36.  It  is  not  definitely   known  whether  alloys  are 
mixtures  or   chemical    combinations  of  their  in- 
gredient metals.    Electrically,  it  might  be  supposed  from 
the   resistivity  of   alloys,  that  in  some  cases  alloys  are 
mere  mixtures,  and   in   others   chemical  combinations. 
Thus,   alloys   of    such   metals   as   lead,    tin,    zinc,   and 
cadmium,  behave  electrically  like  bundles  of  wire  made 
up  in  the  proportions  of  their  respective  metals,  while 
alloys   of    such    metals   as  gold,    silver,    copper,    iron, 
aluminium,  and  others,  give  a  resistivity   much  higher 
than  that  which  a  mere  bundle  of  such  wires  would  lead 
one  to  expect. 

The  temperature  coefficient  of  an  alloy  is  always  less 
than  would  be  expected  from  the  temperature  coefficients 
of  its  ingredients,  and  the  greater  the  resistivity  of  an 
alloy,  the  lower  will  usually  be  its  temperature  co- 
efficient. Thus  german  silver  has  a  resistivity  of  about 
21  microhms  (varying  considerably  with  different 
samples)  and  a  temperature  coefficient  of  about  0.044 
per  cent,  per  degree  C.;  while  platinoid  has  a  resistivity 
of  32.7  microhms,  (also  varying  greatly  with  different 
samples),  and  a  temperature  coefficient  of  about  0.021  per 
cent,  per  degree  C. 

37.  It  has  recently  been  shown  that  at   very    low 
temperatures  certain  pure  metals  have  exceedingly 

low  resistivities  at  the  lowest  temperature  at  present 
experimentally  attainable  ( — 197°  C),  and  it  has  been 
inferred  from  such  observations  that  at  the  assumed 
temperature  of  absolute  zero  ( — 273.6°  C.)  the  resistivity 
of  such  metals  would  be  zero,  so  that  a  copper  wire  at 
such  a  temperature  would  have  no  resistance  whatever. 
It  would  appear,  however,  from  what  has  already  been 


27 


mentioned  concerning  alloys  and  their  lower  temperature 
coefficients,  that  their  resistivities  would  still  be  consid- 
able,  even  at  the  absolute  zero  of  temperature. 

For  the  same  reasons  alloys  are  greatly  to  be  preferred 
to  pure  metals  for  the  construction  of  resistance 
standards,  which  should  be  as  nearly  constant  as  possible, 
in  order  that  variations  of  temperature  may  make 
the  least  change  in  the  value  of  their  resistances. 

38.  The  materials  forming  the  earth's  crust,  such  as 
clay,  sand,  gravel,  marl,  etc.,  have,  when  dry, 
very  high  resistivities;  but  since  the  ground  below  a 
certain  depth,  is  almost  invariably  moist,  even  in  dry 
weather,  and  since  the  water  contains  various  salts  in 
solution,  the  resistivity  of  the  entire  mass  is  greatly  re- 
duced. When,  therefore,  a  telegraph  conductor  is 
carried  on  poles  between  two  distant  points,  it  is  not 
necessary  to  have  a  second  or  return  wire  to  complete 
the  circuit,  since  the  ground  between  the  two  stations 
may  be  used  as  a  return  conductor,  introducing  a  resist- 
ance much  less  than  that  which  a  metallic  return  con- 
ductor would  possess.  This  is  due  to  the  enormous  area 
of  cross  section  of  the  earth,  which  is  so  great  that  the 
difference  in  the  earth's  resistance,  as  measured  between 
two  terminals  one  mile  apart,  or  one  hundred  miles 
apart,  is  practically  imperceptible. 

In  order  to  ensure  sufficient  contact  with  the  ground, 
the  ends  of  the  ground  wires  are  connected  with  large 
metallic  plates  called  ground  plates,  usually  of  copper 
or  iron,  and  sometimes  surrounded  by  charcoal,  buried 
sufficiently  deeply  to  meet  permanently  moist  strata.  In 
cities,  the  gas  or  water  pipes,  from  their  extended  buried 
surfaces,  serve  as  excellent  telegraph  ground  plates.  The 


t 


Of  THB 

tJlUVBRSITY 

/?^ 


28 


resistance  of  the  ground  in  a  circuit  may  vary  from  a 
fraction  of  an  ohm  to  hundreds  of  ohms,  according  to 
the  nature  of  the  ground  connections,  such  high  resist- 
ances being  met  with  in  cases  where  the  ground  is 
improperly  made,  or  where  the  strata  in  which  the  plates 
are  buried  are  permanently  dry.  Thus  arise  two 
varieties  of  circuits,  one,  the  metallic  circuit,  in  which 
the  circuit  is  metallic  throughout;  and  the  other,  the 
ground  return  circuit,  in  which  the  ground  is  used  as 
the  return  conductor. 


FIG.  9. — VARIETIES  OF  INSULATORS 


39.  On  all  pole  lines  the  conductor,  whether  cover- 
ed or  bare,  is  supported  on  suitable  insulators. 
Fig.  9  shows  a  variety  of  insulators  differing  in  shape 
and  material.  A,  is  a  glass  insulator;  B,  an  oilcup  in- 
sulator; c,  a  hard  rubber  insulator;  and  D,  a  porcelain 
insulator.  These  insulators  are  rigidly  supported  on 
pins  placed  on  cross-arms.  The  resistivity  of  all  these 
insulating  materials  is  very  high  and  is  most  con- 
veniently rated  in  quegohms.  The  resistance  of  any 
ordinary  insulator,  between  the  surface  of  the  groove  in 
which  the  wire  is  placed  and  the  surface  of  the  support- 
ing pin,  would  not  be  less  than  500  begohms,  but  in 
practice  the  resistance  an  insulator  offers  to  the  escape  of 


a  current  is  far  less,  owing  to  the  conductance  of  a  film 
of  moist  dust  and  dirt  on  its  surface.  This  is  especially 
true  of  glass,  on  account  of  its  hygroscopic  nature,  and 
these  insulators  are,  therefore,  not  well  adapted  for  use 
in  a  moist  climate.  The  advantage  of  an  oil  insulator 
arises  from  the  fact  that  the  oil  interposes  a  high  resist- 
ance path  to  leakage  over  the  surface,  dust  and 
moisture  settling  to  the  bottom  of  the  oil,  leaving  a  clean 
surface. 

The  greater  the  number  of  insulators  supporting  a  wire, 
the  greater  the  number  of  conducting  paths  for  the 
escape  of  current  to  ground,  and  hence  the  greater  the 
leakage.  Therefore,  the  greater  the  length  of  conduct- 
ing circuit,  the  greater  the  leakage,  and  the  smaller  its 
insulation  resistance.  The  apparent  insulation  of  a  line 
measured  in  megohms,  multiplied  by  its  length  in  miles, 
gives  its  average  apparent  insulation  per  mile  in 
megohm-miles. 

40.  Resistances  in  various  forms,  usually  in  coils  of 
wire,  are  introduced  in  working  circuits  for  the 
purpose  of  controlling  or  limiting  the  current  strength. 
In  other  cases  they  -are  introduced  for  purposes  of 
measurement.  In  all  cases,  however,  the  conditions 
must  be  such  that  the  current  passing  through  them  shall 
not  produce  excessive  heating. 

For  variable  resistances,  such  as  are  employed  for  con- 
trolling the  current  strength  in  working  curcuits,  iron 
wires,  strips  or  plates,  carbon  blocks  or  discs,  or  columns 
of  liquid  are  employed,  so  arranged  that  different 
lengths  can  be  readily  introduced  or  removed  from  the 
circuit. 

Figs.  10  and  10#  shows  such  an  arrangement,  where,  by 


30 


the  movement  of  the  lever  arm,  the  contact  strip  c,  can  be 
brought  into  contact  with  any  of  the  metal  buttons  from 


/*«•£*«<*«••/'    W  W  Klec.l 

FIGS.  10  AND  IQa. — RESISTANCE  FRAME  AND  DIAGRAM  OF  CONNECTIONS. 

A  to  B,  thus  varying  the  number  of  coils  of  wire  in  the 
circuit.     The  particular  instrument  shown  is  designed  to 


FIG.  11. 

carry  the  current  to  supply  six  ordinary  incandescent  16 
candle-power,  110- volt  lamps,  without  overheating. 


Fig.  11  shows  a  variable  resistance  or  rheostat  formed 
of  iron  wire,  embedded  in  porcelain  enamel,  in  firm  con- 
tact with  the  heavy  iron  bed-plate.  This  arrangement 


FIG.  12.— WHKATSTONE  BRIDGE. 

ensures  a  rapid  transference  of  the  heat  generated  by  the 
current  in  the  wire  to  the  iron  plate,  and  its  subsequent 
diffusion  and  radiation. 

41.  Where  resistances  are  employed  for  the  purpose 
of  comparison  or  measurement,  their  values  in 
ohms  are  calibrated  with  reference  to  a  standard  ohm. 
In  this  instrument  a  coil  of  platinum-silver  alloy  having 
the  resistance  of  one  ohm  at  some  convenient  definite 
temperature,  has  its  terminals  connected  to  two  stout 


t  Uc.  Engineer 

FIG.  13. — DIAGRAM  OF  CONSTRUCTION  OF  RESISTANCES  IN  BRIDGE. 
copper  rods  whose  ends  dip  in  mercury  cups.     The  coil 
and  rods  within  the  case  are  embedded  in  paraffin  wax, 
a  central  hollow  core  being  left  for  the  insertion  of  a 


32 


thermometer,    when    the   instrument   is   submerged    in 
water  or  oil. 

Figs.  12  and  13  represents  a  common  form  of  resist- 
ance box  called  a  Wheatstone  bridge  or  Wheat-stone 
balance.,  a  plan  of  which  appears  beneath  together  with 
a  diagramatic  view  of  the  coils  and  plugs.  By  suitable 
combinations  of  plugs,  the  resistance  included  between 
the  terminals  x  and  r,  can  be  made  any  integral  number 
of  ohms  between  zero  and  10,000. 
SYLLABUS. 

The  conductivity  of  a  material  is  the  reciprocal  of  its 
resistivity. 

The  conductance  of  a  conductor  or  circuit,  is  the 
reciprocal  of  its  resistance,  and  is  measured  in  mhos. 

The  total  resistance  of  a  number  of  resistances  in 
series,  is  the  sum  of  those  resistances,  and  the  total  con- 
ductance of  a  number  of  conductances  in  parallel  is  the 
sum  of  those  conductances. 

The  temperature  coefficient  of  alloys  is  less  than  the 
temperature  coefficient  of  pure  metals. 

Metallic  circuits  are  metallic  throughout.  Ground 
return  circuits  complete  their  circuit  through  the  earth's 
substance. 

The  resistance  of  insulators  is  in  practice  the  resistance 
of  a  film  of  dirt  or  moisture  upon  their  surfaces.  The 
insulation  of  a  line  is  expressed  in  megohm-miles  repre- 
senting the  average  insulation  resistance  of  a  single  mile. 

Resistances  for  measurement  are  usually  in  coils  of 
wire  of  a  suitable  alloy.      Resistances  for  controlling 
current  strength  in  working  circuits  are  frequently  of 
iron,  carbon  or  water. 
Laboratory  of  Houston  &  Kennelly, 
Philadelphia. 


[Copyright,  1894,  by  THE  ELECTRICAL  ENGINEER.] 
WEEKLY. 


.  5.  si 

Electrical    Engineering   Leaflets, 


Prof.  E.  J.  Houston,  Ph.  D. 

AND 

A.  E.  Kennelly,  F.  R.  A.  S. 


I  NTERIVl  EDI  ATE     GRADE 

ELECTRIC 


42.  The  Wlieatstone  bridge  shown  in  Figs.  12  and 
13,  (No.  4,  Intermediate  Grade)  is  used  for 
determining  the  resistance  of  any  conducting  path  or 
circuit.  The  electrical  connections  of  the  bridge  are 
shown  in  Fig.  14,  where  E,  is  an  E.  M.  F.,  usually  a  battery, 
connected  to  the  terminals  q  and  r.  The  current  from  E, 
divides  between  the  paths  q  x  r  and  q  z  r,  where  q  x,  and 
q  z,  are  resistances,  usually  called  the  arms  of  the  bridge 
or  balance,  x  /*,  is  the  adjustable  and  known  resistance 
under  the  plugs,  while  z  r,  is  the  unknown  resistance  to 
be  measured.  Calling  the  pressure  at  r,  zero,  and  at  ^, 
E,  volts,  as  shown  in  Fig.  15,  the  fall  of  pressure  through 
the  resistances  will  be  shown  by  the  inclined  lines  p  a  r, 
and  P  1)  r.  It  is  evident  that  if  the  resistances  q  x  and  q  2 
are  equal,  and  also  x  r  and  z  r,  then  by  symmetry  the 
pressure  x  #,  Avill  be  equal  .to  the  pressure  z  b.  Con- 
sequently, if  these  points  x  and  z,  are  connected  through 
a  galvanometer,  or  instrument  for  detecting  a  current, 

Published  by 

THE   ELECTRICAL  ENGINEER, 
203  Broadway,  New  York,  N.  Y. 

[Entered  as  second-class  matter  at  the  New  York,  N.  Y.,  Post  Office,  June  14,  1894.] 


34 


no  current  will  pass,  and  the  galvanometer  needle  will 
remain  at  zero.  The  resistance  in  each  of  the  arms  q  x 
and  q  2,  are  usually  10,  100,  or  1000  ohms,  and  in  making 
a  measurement  the  plugs  in  the  branch  x  r,  are  removed 
or  replaced  until  the  galvanometer  shows  no  current 
passing.  When  this  is  the  case  the  unknown  resistance 
z  /*,  is  equal  to  the  resistance  unplugged  in  <c  /',  provided 
that  q  x  and  q  z,  are  equal.  If  q  2,  bears  any  other  pro- 
portion to  q  x,  then  z  /*,  bears  the  same  proportion  to  the 
unplugged  resistance  x  r. 


FIG.  14. — DIAGRAM  OF  WHEAT- 
STONE  BRIDGE. 


FIG.  15.—  DIAGRAM  OF  FALL  OF 
PRESSURE  ix  BRANCHES  OF 
WIIEATSTOXE  BRIDGES. 


43.  In  order  accurately  to  define,  for  practical  or 
commercial  purposes,  the  conductivity  of  copper 
wire,  a  standard  has  been  very  generally  agreed  upon 
based  upon  the  researches  of  Matthiessen,  and  called 
Matthiesserf  s  standard  of  conductivity.  The  resistivity 
of  soft  copper  of  this  standard  quality  is  1.594  microhms 
at  0°  C.,  although  copper •  has  been  obtained  at  4  per 
cent,  higher  conductivity  than  that  of  Matthiessen's 
standard.  A  soft  copper  wire  of  Matthiessen's  standard 


35 


conductivity,  one  metre  long  and  weighing  one  gramme, 
will  have  a  resistance  of  0.14173  international  ohms  at 
0°  C.  It  is  common,  however,  in  specifications,  to  call 
for  a  conductivity  in  copper  of  not  less  than  96  per  cent, 
of  Matthiessen's  standard.  The  following  is  a  table  of  the 
resistance  of  ordinary  sizes  of  soft  copper  wire  of 
Matthiessen's  standard  conductivity  at  20°  C.  Hard 
drawn  copper  has  a  conductivity  of  about  2J  per  cent, 
less  than  soft  annealed  copper. 


Brown  and 
Sharp 
Guage  No. 

Diameter 
Inch. 

Lbs.  per 
Foot. 

Lbs. 
Mile. 

Ohms  per 
Foot. 

Ohms 
Mile. 

0000 

0.460 

0.6405 

3382 

0.00004893 

0.258 

UOO 

0.4096 

0.5080 

2682 

0.00006170 

('.326 

00 

0.3ti48 

0.4o28 

2127 

O.i  00o7780 

o.4ri 

0 

0.3249 

03195 

1687 

0.00009811 

0.518 

•     1 

0.2893 

0.2533 

1337 

0.0001237 

0.653 

2 

02576 

0.2>i09 

1061 

0.0001560 

0.824 

3 

0.2294 

0.1593 

841 

0.0001967 

1.1-39 

4 

0.2043 

0.1264 

667 

0.0002489 

1.309 

5 

0.1819 

0.1002 

529 

0.0003128 

1.651 

6 

0.  1  620 

0.07946 

420 

O.OOC3944 

2.082 

7 

01443 

0.06302           333 

0.0004973 

2.626 

8 

0.1285 

0.04998 

264 

O.(00627l 

3.311 

9 

0.1144 

0  03963 

2o9 

0.0007908 

4.176 

10 

0.1019 

0.03143 

166 

O.OOt.9972 

5.265 

11 

0.091)74 

0.02493 

132 

O.(0l257 

6636 

12 

0.08081 

0.01977 

104 

0.001586 

8.374 

44.  When  two  "conducting  surfaces  are  placed 
lightly  in  contact,  the  resistance  at  the  contact  is 
much  greater  than  if  the  surfaces  are  pressed  firmly  to- 
gether. This  is  due  to  the  fact  that  under  light  pressure, 
even  in  the  case  of  the  smoothest  surfaces,  the  points  of 
contact  are  comparatively  few,  and  as  the  pressure  in- 


creases,  these  contact  points  are  increased,  thus  increas- 
ing the  available  cross-section  of  contact. 

Moreover,  iilms  of  oxide,  the  resistivity  of  which  is 
comparatively  high,  and  which  are  apt  to  form  rapidly 
on  nearly  all  metallic  surfaces,  increase  the  difficulty  of 
obtaining  perfect  contact.  Consequently,  care  must  be 
exercised,  when  introducing  apparatus  into  circuits,  that 
the  contact  surfaces  are  free  from  oxide  or  grease,  and 
are  brought  firmly  together,  especially  in  cases  where 
the  introduction  of  additional  resistance  is  deleterious. 

An  excellent  form  of  variable  resistance  is  based  upon 
the  preceding  principle  of  resistance  offered  by  light 
contacts ;  namely,  the  carbon  rheostat.  This  rheostat 
consists  essentially  of  disks  or  plates  of  carbon,  piled 
together,  and  provided  with  suitable  means  for  varying 
their  pressure.  The  telephone  transmitter  in  common 
use  employs  the  principle  of  variable  contact  pressure, 
to  impress  on  the  telephone  conducting  line,  variations 
in  the  current  strength  corresponding  to  the  vibrations 
of  sound. 

Nearly  all  the  telephone  transmitters  in  common  use 
employ  either  the  principle  of  variable  contact  pressure, 
or  the  principle  of  varying  the  length  and  cross-section 
in  conducting  path  through  carbon  particles,  vibrating 
between  relatively  non-vibratory  electrodes. 

45.  When  the  current  strength  remains  constant, 
the  resistance  of  any  part  of  the  circuit  depends  natur- 
ally upon  the  resistivity  of  that  part,  its  dimensions 
and  temperature.  When,  however,  the  current  strength 
varies  rapidly,  the  form  or  shape  of  the  conductor  also 
affects  its  resistance.  For  instance,  a  stranded  conduc- 


tor,  that  is,  a  conductor  formed  of  a  number  of  separ- 
ate wires  layed-up  together,  offers  an  apparently  lower 
resistance  to  a  rapidly  alternating  current,  that  is,  to  a 
current  rapidly  changing  its  direction,  than-  would  a  solid 
conductor  of  the  same  length  and  total  cross-section. 
The  cause  of  this  increase  of  resistance  will  be  con- 
sidered in  a  subsequent  leaflet. 

-tH.       It  is  now    well  recognized   that   lightning  dis- 
charges partake  of  a  rapidly  alternating  character. 
An  advantage  is  therefore  secured  from  the  use  of  a 
stranded  or  strip  lightning  rod,  as  opposed  to  a  solid  rod 
of  the  same  weight  per  unit  of  length. 

-t7.       The  following  is  a  table  of   the  resistances  of 
various   apparatus   employed  in    the  commercial 
applications  of  electricity. 

GALVANOMETERS. 

Thomson  Mirror  Galvanometer 1  ohm  to  350,000  ohms. 

"  "      Common  resistance 5,000      '* 

Thomson  Marine  Galvanometer 5,000  to  50,000 

D' Arson val  Galvanometers         1  ohm  to  750 

'*          Common  resistance 250      <k 

AMMETERS. 

Resistance    usually    inversely    as   maximum    current 
strength  measured. 

Weston,  15  amperes 0.0022  ohms. 

Kelvin  Balances,  Centiampere  balance 160  ohms. 

"  "          Deciampere  balance .2      " 

"  Ampere  balance 0.18  ohm. 

"  "         Composite  balance  as  wattmeter  or  centi- 

ampere  balance 30  ohms 

'•  "          as  voltmeter    (for    voltages  up    to    200 

volts) 200  to  500      " 


38 


VOLTMETERS. 

Cardew  voltmeters  for  100  volts,  about 500  ohms. 

Weston  voltmeters  for  150  volts,  about 19,000      " 

Thomson  Marine  voltmeter  for  120  volts 1,000      " 

Weston  alternating  current- volt  meters  for  120  volts 

2,500  to  3,500      " 

BATTERIES. 

Callaud  gravity  cell 2  to  4  ohms. 

Leclanche 1  ohm. 

Bichromate 0.4        " 

Edison-Lalande,  300  ampere-hours 0.02      " 

Storage  cell.  100  ampere-hours 0.005     " 

TELEGRAPHY. 

Sounders 0.5  to  20 

Neutral  relays 80  to  300,  usually  150 

Polar  relays 100  to  500,  usually  about  400 

FIRE  TELEGRAPHY. 

Gongs,  about 20  ohms. 

Signal  box  magnets,  about 8      " 

SUBMARINE  TELEGRAPHY. 

Speaking  mirror. .  1,000  to  5,000  ohms. 

Usually 2,500      " 

Siphon  recorder  coils,  about 500 

TELEPHONY. 

Bell  telephone,  about 75  ohms. 

Call  bell,  from 75  to  1,000      " 

Magneto-armature 500 

Induction  coil,  primary 0.28      " 

Induction  coil,  secondary 12  to  160 

DYNAMOS  AND  MOTORS, 

For  a  given  power,  or  output  of  these  machines,  the 
armature  resistance  varies  approximately  as  the  square 
of  the  E.  M.  F.,  and  for  any  E.  M.  r.,  roughly  in  inverse 
ratio  to  the  power. 


39 


Armature  resistance  (warm)  of  0.5  K.  w.  dynamo  or  motor  about  4  ohms. 

And  between  brushes  3.  •'       0.4        " 

20         "  ••       0.025     " 

100         "  "  "       O.OU55  " 

200        '•  (<       0.0024  " 

ALTERNATING  CURRENT  TRANSFORMERS. 
Resistances  of  primary  and  secondary  coil  vary  with 
frequency  of  alternation  and  a  variety  of  other  circum- 
stances.    Roughly,  they   vary  inversely    as   the   power 
rating  of  the  machine.    The  following  are  a  few  examples. 

0.5     K.  w.  primary  21.8  ohm,        Secondary  0.04      ohm. 
2  "          "  5.5  ohms  *  0.015       " 

20  "          "  0.48  ohm  •'  0.0015     •' 

INCANDESCENT  LAMPS. 

The  resistance  varies  approximately  inversely  as  candle- 
power  when  operated  at  uniform  E.  M.  F. 

Thus  at  115  volts  an   8  candle-power  lamp,  hot,  500  ohms. 

10       "         '•  "        "  406     " 

16       M         "  "        "  254     " 

32       "         "  "       "  127     '• 

The  human  body  varies  enormously  in  its  resistance 
with  the  position  of  electrodes,  their  surface  area,  the 
dryness  or  moisture  of  the  skin,  the  duration  of  the  ap- 
plication, and  the  current  strength.  Lowest  resistance 
on  record,  214  ohms  from  surface  of  head  to  surface  of 
right  calf,  and  500  ohms  from  hand  to  hand  each  im- 
mersed to  wrist  in  salt  water.  Average  resistance  under 
latter  conditions  1000  ohms. 

The  resistance  of  the  lightning  rod  on  the  Washington 
monument  (550  feet  high)  including  ground  connections 
is  2.3  ohms. 

SYLLABUS. 

In  a  Wheatstone  bridge,  the  value  of  an  unknown 
resistance  is  determined  by  equalizing  the  potential  at 


40 


two  definite  points  in  a  divided  circuit,  one  branch  con- 
taining an  adjustable  resistance,  and  the  other  the 
unknown  resistance. 

Matthiessen's  standard  of  conductivity  is  based  upon 
measurements  made  by  Matthiessen  of  the  resistivity  of 
the  purest  copper  he  was  able  to  obtain. 

Hard  drawn  copper  has  a  conductivity  of  about  i^per 
cent,  less  than  annealed  copper. 

The  resistance  of  contacts  between  surfaces  depends 
upon  the  nature  of  the  surfaces,  their  area,  and  the 
pressure  with  which  they  are  brought  together. 

Since  the  resistivity  of  films  of  metallic  oxides  is  high, 
the  presence  of  such  films  should  be  carefully  avoided 
in  jointing  conductors. 

All  carbon  rheostats  and  some  telephone  transmitters 
utilize  the  principle  of  variable  contact  pressure. 

A  stranded  conductor  offers  an  apparently  lower 
resistance  to  a  rapidly  alternating  current  than  would  a 
solid  conductor  of  the  same  length  and  weight.  Con- 
sequently lightning  conductors  should  be  preferably 
stranded,  lightning  discharges  being  usually  oscillatory. 

Laboratory  of  Houston  &  Ken  nelly, 
"Philadelphia. 


[Copyright,  1894,  by  THE  ELECTRICAL  ENGINEER.] 
WEEKLY. 


Price,     -    10  Cents. 
Subscription,  $8.00. 


Electrical   Engineering   Leaflets, 


Prof.  E.  J.  Houston,  Ph.  D. 

AND 

A.  E.  Kennelly,  F.  R.  A.  S. 


INTEI3JYIEDIATE     GRADE. 

ELECTRIC  CURRENT. 


48.  It  is  evident  that  the  effects  produced  by  the 
passage  of  electricity  in  a  circuit  are  dependent 

on  the  time  during  which  the  flow  is  maintained.  For 
example,  the  amount  of  work  done  by  an  electric  motor 
depends  entirely  upon  the  current  which  is  passing 
through  the  motor,  and  also  upon  the  time  during  which 
the  current  is  supplied.  Again,  the  amount  of  heat  and 
light  emitted  by  an  electric  lamp  depends,  for  a  given 
current,  upon  the  time  during  which  the  lamp  is  lighted. 
The  amount  of  metal  deposited  in  a  plating  bath  depends 
similarly,  for  a  given  current,  upon  the  time  during  which 
the  current  is  supplied. 

Any  of  these  electric  effects,  therefore,  must  be  attri- 
buted to  the  passage  of  a  definite  quantity  of  electrical 
now,  maintained  for  a  definite  time. 

49.  Just  as  the  flow  of  water  through  a  pipe  may  be 
correctly  rated  as  a  certain  number  of  cubic  inches 

per  second,  so  the  flow  of  electricity  through  a  conductor  or 

Published  by 

THE   ELECTRICAL  ENGINEER, 
203  Broadway,  New  York,  N.  Y. 

[Entered  as  second-class  matter  at  the  New  York,  N.  Y.,  Post  Office,  June  14,  1894.") 


circuit  may  be  correctly  rated  as  a  certain  number  of 
coulombs  of  electricity  per  second.  A  coulomb  is,  there- 
fore, the  unit  quantity  of  electricity,  and  corresponds  in 
cases  of  electric  current  to  the  unit  quantity  of  water  in 
hydraulics,  say,  a  cubic  inch. 

Since  electricity  is  invisible,  a  coulomb  cannot  be  seen. 
It  can,  however,  be  rigorously  measured  by  its  properties. 
For  example,  a  coulomb  of  electricity  on  passing  through 
a  chemical  solution,  liberates  by  decomposition  perfectly 
definite  quantities  of  the  constituent  elements  of  that 
solution.  For  example,  a  coulomb  will  deposit  1.118 


FIG.  16. — ELECTROLYTIC  EFFECT  OF  THE  ELECTRIC  CURRENT. 
milligrammes  of  silver  from  a  solution  of  a  salt  of  silver, 
or  will  liberate  0.01038  milligramme  of  hydrogen  from 
water.  So  rigorously  are  these  relations  maintained,  that 
for  many  years  the  voltameter  shown  in  Fig.  16,  was 
almost  the  only  instrument  for  measuring  electric  current. 
The  current  from  the  battery  B,  enters  the  acidulated 
water  in  the  glass  vessel  vv,  through  platinum  electrodes 
at  the  base  connected  with  the  leading-in  wires  marked 
-\-  and  — .  The  passage  of  the  electricity  causes  the 
water  to  be  decomposed,  and  causes  hydrogen  and  oxygen 
to  be  liberated  at  the  negative  and  positive  electrodes  re- 
spectively, and  the  gases  evolved  collect  into  the  test  tube 
placed  over  the  platinum  plates,  thus  displacing  the  water 


with  which  the  tubes  were  already  filled.  Since  one 
milligramme  of  hydrogen  at  the  standard  temperature  and 
pressure  (760  cms.  of  mercury  and  0°C.)  occupies  11.16 
c.  c.,  the  reading  of  the  level  of  liquid  in  the  graduated 
tubes  will  show,  when  corrected  for  temperature  and 
pressure,  the  total  number  of  coulombs  that  have  passed 
through  the  apparatus. 

50.  A  current  of  electricity  is  not  a  total  flow,  but 
a  rate  of  flow,  and  this  distinction  must  be  care- 
fully borne  in  mind.  Thus  the  voltameter  just  described 
does  not  measure  current  strength  directly.  What  it 
measures  is  the  total  quantity  of  electricity  which  passes 
in  coulombs,  and,  therefore,  it  has  to  be  regarded  as  a 
coulomb-meter 

The  unit  rate  of  flow  in  electricity  is  called  an  ampere, 
after  a  celebrated  French  electrician,  Ampere.  It  has  a 
rate  of  flow  equal  to  one  coulomb  of  electricity  per 
second;  and,  just  as  the  rate  of  flow  of  water  through  a 
pipe  may  be  measured  in  cubic  inches  per  second,  so  the 
flow  of  electricity  through  a  conducting  circuit,  or  its  cur- 
rent strength,  may  be  measured  in  amperes,  or  coulombs- 
per-second. 

The  International  ampere,  so  called  because  its  value 
is  adopted  by  all  civilized  nations,  is  practically  defined 
as  that  current  which  when  passed  through  a  properly 
prepared  aqueous  solution  of  silver  nitrate  will  liberate 
1.118  milligrammes  of  silver  per  second.  This,  as  we 
have  already  seen,  is  the  amount  deposited  by  a  coulomb 
in  any  time,  and  the  ampere  is  to  deposit  this  in  one 
second,  since  one  second  is  required  for  the  passage  of 
one  coulomb  under  this  current  strength. 


44 


51.  When  an  ampere  flows  steadily  through  a  circuit 
during  one  hour,  the  total  quantity  of  electricity 

which  passes  is  3600  coulombs,  this  being  the  number  of 
seconds  in  an  hour.  An  ampere-hour  is,  therefore  a  unit 
of  electric  quantity  equal  to  3600  coulombs.  This  unit  is 
in  common  use.  112.7  ampere-hours  are  required  for 
the  decomposition  of  a  pound  of  silver.  Similarly  374.3 
ampere-hours  are  required,  for  the  decomposition  by 
electro-plating  of  a  pound  of  zinc.  Again  in  a  voltaic 
cell  one  pound  of  zinc  will  furnish  a  quantity  of  elec- 
tricity equal  to  1,347,500  coulombs  or  374.3  ampere- 
hours,  assuming  that  there  is  110  loss  of  zinc  by  local 
action.  If  a  battery  of  ten  cells  in  series  delivers  374.3 
ampere-hours  collectively,  one  pound  of  zinc  will  be 
burnt  in  each  cell. 

52.  Electric    currents   may   be   classed   into   three 
different  varieties. 

(1.)  The  continuous  current.  (2.)  The  pulsatory  cur- 
rent. (3.)  The  alternating  current. 

The  continuous  current  has  constant  direction  and 
magnitude,  or,  if  varying,  does  not  vary  periodically. 
Such  currents  are  generally  obtained  from  thermopiles, 
voltaic  or  primary  cells,  and  storage  or  secondary  cells. 

The  pulsatory  current  is  one  in  which  the  direction  is 
uniform  but  the  strength  varies. 

Pulsatory  currents  are  practically  used  in  many  forms 
of  signal  and  telegraphic  apparatus,  also  in  some  arc 
light  and  power  circuits.  Strictly  speaking  nearly  all 
continuous  current  dynamos  furnish  pulsatory  rather  than 
continuous  currents,  owing  to  the  slight  fluctuations  pro- 
duced at  the  changes  of  the  commutator  segments  under 
the  brushes.  In  well  designed  continuous  current  dyna- 


45 


mos,  however,  these  pulsations  are  usually  so  slight,  that 
the  current  they  furnish  may  be  considered  as  practically 
continuous.  But  even  in  the  best  designed  continuous 
current  dynamo,  except  in  those  of  the  so-called  unipolar 
type,  these  pulsations  do  exist. 

Alternating  currents  are  those  whose  direction  per- 
iodically alternates,  and  whose  current  strength  is  also 
periodically  variable.  They  are  extensively  employed 
in  electric  lighting  and  in  the  transmission  of  power. 

53.  The  following  is  a  table  of  current  strengths 
employed  in  various  practical  applications  : 

The  aggregate  maximum  current   strength  de- 
livered by  all  the  dynamos  lighting  New  York  bo 
City  is  about ~T&£ kilo-amperes. 

The  strength  of  current  employed  in  electric 

welding  is  often .20  to  50  kilo-amperes. 

The  current  strength  usually  employed  in  arc 
lighting  is 8  to  10  amperes. 

The  current  strength  required  to  operate  the 
average  110- volt,  16  c.  p.  lamp  is  (either  alter- 
nating or  continuous) .  .  0.45  ampere. 

The  current  strength  required  to  operate  the 
average  telegraph  circuit  is 25  to  35  milli-amperes. 

The  minimum  current  stated  to  be  appreciable 

by  the  Bell  telephone 0.6  tricro-ampere. 

The  minimum  current  practically  appreciable  by 
Thomson  mirror  galvanometer  (one  scale  di- 
vision of  a  semi-millimetre  at  a  distance  of 
one  metre),  about 20  tricro-amperes. 

Alternating  current  strength  hitherto  employed 
in  the  execution  of  criminals  by  electricity 
(New  York  State) 3  to  8  amperes. 

The  average  current  strength  employed  in  firing 
electric  fuses,  about. 0.5  ampere. 

54.  The  capability  of  electricity  to  produce  chemical 
decomposition  is  very  seldom  practically  employed 

for  purposes  of  measuring  the  current  strength  in  a 
circuit.  It  is  much  more  convenient  to  employ  the  mag- 
netic effect  produced  by  the  current.  When  an  electric 


current  traverses  a  conductor,  it  produces  in  the  space 
surrounding  the  conductor  what  is  called  a  magnetic  field, 
that  is,  a  space  permeated  by  magnetism.  When  a  mag- 
netizable substance  is  brought  into  this  magnetic  field,  the 
passage  of  the  flux  through  it  tends  to  give  it  a  set  or 
direction,  which  is  more  marked  when  the  magnetizable 
substance  possesses  a  magnetism  of  its  own ;  i.  e.9  is 
already  magnetized.  A  magnetizable  substance  is  gene- 
rally made  in  the  form  of  a  magnetic  needle,  or  it  may 


FIG.  17. — WESTON  AMMETER.        FIG.  18. — THOMSON  MIRROR  GAL- 
VANOMETER, TRIPOD  FORM. 

be  replaced  by  a  movable  coil  or  conductor  which  pos- 
sesses a  magnetic  field  when  traversed  by  an  electric 
current.  Instruments  of  this  character  employed  for  the 
measurements  of  electric  currents  are  called  galva- 
nometers, amperemeters,  or  ammeters.  A  few  galvanom- 
eters are  shown  in  the  figures. 

Fig.  17  shows  an  ammeter  called  a  Weston  ammeter. 
Here  a  permanent  magnet  is  employed  and  a  coil  of  wire 


capable  of  rotation  between  its  poles,  is  moved  by  the 
measured  current  against  the  action  of  a  spring.  A 
pointer,  suitably  fixed  to  the  coil,  indicates  upon  the  scale 
the  current  strength. 

Fig.  18  shows  a  common  form  of  Thomson  mirror  gal- 
vanometer mounted  on  a  tripod.  The  current  to  be 
measured  passes  through  a  circular  coil  in  the  instrument, 
at  the  centre  of  which  is  a  small  magnetic  needle  sus- 
pended on  a  silk  fiber,  and  attached  to  the  back  of  a 
glass  mirror.  The  controlling  magnet  M,  is  so  supported 


FIG.  19. — THOMSON  MIRROR  GALVANOMETERS,  LAMP  AND  SCALE. 
on  a  vertical  rod  that  it  can  be  moved  up  or  down  the  rod, 
and  also  rotated  by  the  thumbscrew  s.  The  position  of 
this  magnet  controls  the  position  of  the  magnetic  needle, 
and  also  the  sensibility  of  the  instrument.  When  a  cur- 
rent passes  through  the  coil,  it  causes  the  magnetic  needle, 
with  its  mirror  to  deflect,  and  a  beam  of  light  from  a 
suitably  placed  lamp  is  reflected  back  upon  a  scale.  The 
movement  of  the  reflected  image  or  spot  of  light  on  this 
scale  measures  the  current  strength. 


4S 


Fig.  19  shows  two  other  forms  of  Thomson  mirror  gal- 
vanometer, in  which  two  superposed  coils  of  wire  are  em- 
ployed with  a  double  magnet  system  and  mirror  attached. 
This  arrangement  which  is  called  an  astatic  system, 
secures  a  high  degree  of  sensibility.  A  convenient  form 
of  scale  for  use  with  such  instruments,  is  shown  on  the 
right. 

SYLLABUS. 

The  effects  produced  by  an  electric  current  depend 
upon  the  time  during  which  the  current  is  passing 
through  the  circuit, 

The  flow  of  water  through  a  pipe  may  be  conveniently 
rated  in  cubic  inches  per  second. 

The  flow  of  electricity  through  a  conductor  may  be 
conveniently  rated  in  coulombs  per  second. 

The  unit  quantity  of  electricity  is  called  a  coulomb. 

A  coulomb  of  electricity  will  liberate  1.118  milli- 
grammes of  silver  from  a  solution  of  a  salt  of  silver,  or 
0.01038  milligramme  of  hydrogen  occupyingftll  16  c.  c., 
at  standard  temperature  and  pressure. 

The  coulombs  passing  in  any  circuit  may  be  measured 
by  a  voltameter. 

The  ampere,  represents,  not  the  total  quantity  of  elec- 
tricity which  has  passed,  but  the  instantaneous  rate  at 
which  it  is  passing,  that  is,  its  rate  of  flow. 

The  international  ampere  is  one  coulomb  per  second. 

The  magnetic  effect  is  the  most  convenient  effect  of  a 
current  to  employ  for  measuring  its  strength,  and  is  used 
in  galvanometers. 

Laboratory  of  Houston  &  Keimelly, 
Philadelphia, 


[Copyright,  1894,  by  THE  ELECTRICAL  ENGINEER.  ") 


WEEKLY. 


Electrical   Engineering   Leaflets, 


Prof.  E.  J.  Houston,  Ph.  D. 

AND 

A.  E.  Kennelly,  F.  R.  A.  S. 


INTERMEDIATE 

OHM'S 


55.  The  current  strength  in  any  continuous  current 
circuit  depends  upon  the  electromotive  force  or 
pressure  m  the  circuit.  If  the  total  electromotive  force 
be  doubled,  then,  other  things  being  equal,  the  current 
strength  will  be  doubled.  In  other  words,  the  current 
strength  is  proportional  to  the  electromotive  force.  The 
current  strength  in  any  continuous  current  circuit  also 
depends  upon  the  resistance  of  the  circuit.  If  the  re- 
sistance be  doubled,  the  current  strength  will  be  halved ; 
in  other  words,  the  current  strength  is  inversely  propor- 
tional to  the  resistance. 

These  effects  were  discovered  by  Dr.  Ohm,  who  an- 
nounced them  in  a  law  generally  known  as  Ohm's  law  ; 
namely  :  The  current  strength  in  a  continuous  current 
circuit  is  obtained  by  dividing  the  total  electromotive 
force  in  the  circuit  by  the  total  resistance  in  the 
circuit. 


Published  by 
THE   ELECTRICAL  ENGINjasi^-*'    Of 

203  K  road  way,  New  V  orkf  NJW^^—  «••  w  w  w  %*v   1*1  v  m  K>M 

SB  8  J  V  2  R  51 T  x 

[Entered  as  second-class  matter  at  the  New  York,  N.  Yt>  lW  Office,  June  14,  1894.] 

.     , 


50 


56.  Ohm's   law  is  generally  expressed  as  follows : 

W 

C  =  -=  ,  in   English  speaking  countries,  where 

C  =  current  strength  in  amperes ;  E  =  E.  M.  F.  in  volts, 
and  R,  the  resistance  in  ohms.  In  foreign  countries  the 
equation  is  usually  written 

7=1 

/,  here  standing  for  the  intensity  of  the  current. 

Since,  however,  at  the  International  Electrical  Con- 
gress at  Chicago  in  1893,  it  was  recommended  to  employ 
a  uniform  symbolic  notation,  in  which  1  was  to  be  used 
internationally  in  place  of  C,  this  latter  symbol  being 
adopted  for  another  purpose,  wre  shall  hereafter  employ 
/,  to  represent  current  strength. 

57.  From  the  formula, 

/=f,  a.) 

we  obtain,  E  =  I R  ,  (2.) 

and  R  =  ^  (3.) 

In  other  words,  having  given  any  two  of  the  three 
essential  quantities,  electromotive  force,  resistance  and 
current  strength,  in  a  continuous  current  circuit,  the  third 
can  always  be  determined  by  the  foregoing  equations. 
These  formulae  are  applicable,  not  only  to  an  entire  cir- 
cuit, but  also  to  any  portion  of  a  circuit;  thus,  the  E.  M.  K. 
of  a  conductor  which  is  required  to  send  through  it  the 
current  it  conveys  is  usually  called  the  "  drop  "  in  that 
conductor. 


51 


58.       For  example,  in  (Fig.  20)  where  a  battery  of 

E.M.F.  E,  and  internal  resistance,  r^  is  represented 

in   circuit  with   an  electromagnet   of   resistance  r2  and 

long  wires  of  total  resistance  rs  leading  to  a  circuit  closer. 

Here  /  =  ^  =  .     -JL 

R       r,+  T%  +  ?'S 

1O  OHMS 


-"c     '  '  ^^ — 

2f  ?i«        **-*!  ih 


85 


I 


P=1OOHMS 


r3=5  OHMS 


O.4-762  AMPERE 


Eltc.Enyineer 

FIG.  20.— DISTRIBUTION  OF  POTENTIAL  DIFFERENCE  IN  A  CIRCUIT. 

in  which  the  resistance  H  is  equal  to  the  sum  of  the 
separate  resistances.  Thus  if  E  —  10  volts,  rv  —  6  ohms, 
r.z  =  10  ohms,  rs—  5ohms,  then  R  =  21  ohms,  and  /=  Jf 
=  0.4762  ampere.  This  current  in  passing  through  eacli 
resistance  rl9  r^  and  rs  is  attended  by  such  a  distribution 
of  E.  M.  F.  as  will  satisfy  equation  (2).  Thus  in  passing 


52 


through  7*2,  it  will  be  attended  by  an  E.  M.  F.  of  *?2 
=  0.4762  X  10  =  4.762  volts,  and  a  voltmeter ,  an  in- 
strument for  measuring  electromotive  forces,  if  suitably 
connected  across  the  terminals  of  rz ;  namely,  between 
a  and  J,  would  indicate  4.762  volts.  The  same  relation 
would  be  true  for  rz,  in  which  there  is  a  drop  of  0.4762 
X  5  =  2.381  volts.  Again  ev  is  the  drop  in  the  battery 
of  0.47H2  X  P>  —  2.857  volts,  so  that  while  the  current 
flows,  the  P.  i>.  at  battery  terminals  —  10  —  2.857  =  7.14o 
volts. 

Since  E  =  el-{-e2  -f-  <?3,  it  is  evident  that  the  total  elec- 
tromotive force  in  a  circuit  is  equal  to  the  sum  of  all  the 
separate  potential  differences,  set  up  in  that  circuit. 


D  D 

FIG.  21.  —  APPLICATION  OF  OHM'S  LAW  TO  A  DIVIDED  CIRCUIT. 

."><».       The  preceding  formulae  are  also  applicable  to 

branch,  derived  or  shunt  circuits.     Thus,  in  Fig. 

21  A,  the  dynamo  E,  whose  E.  M.  F.  is  100  volts,  and  in- 

ternal resistance  r^  is  0.5  ohm,  supplies  three  branches 

/>5,  and  /'6,  of  a^lOgand  ,T0ohms,  respectively,  through 


/» 


4,    5, 


two  leads,  each  of  one  ohm. 

In  this  case  we  may  substitute  for  the  three  resistances 
in  parallel,  their  equivalent  or  joint  resistance  72,  as  in 
Fig.  21  B.  By  what  has  been  already  stated  in  para- 
graph 33,  the  joint  conductance  of  ?'4,  r5,  and  r6,  is  -fa  + 
_i_  _|_  ^_  —  0.05,  and,  therefore,  their  joint  resistance 


is  Tj-.-J-g-  =  20  ohms.     The  total  resistance  in  the  circuit 
is,  therefore,  f/\  +  n  -f-  r$  -f-  R  =  22.5  ohms,  and  the 

^        100 
current  in  the  main  circuit  -«  =  ^x-^  =  4.444  amperes. 

The  drop  in  the  dynamo  armature  due  to  its  resist- 
ance will,  therefore  be  E  =  I R  =  4.444  X  0.5  =  2.222 
volts,  leaving  the  p.  D.  at  dynamo  terminals  97.778  volts. 

The  drop  in  each  lead  will  similarly  be  4.444  volts  or 
8.889  volts  in  both,  leaving  88.889  volts  at  the  terminals 
c.  D. 

O.O1  OHM      O.01  OHM 


FIG.  22.— APPLICATION  OP  OHM'S  LAW  TO  A  CIRCUIT  CONTAINING 
COUNTER  E.  M.  F. 

E    88.889 
The  current  in  r4  will  be  -^= — ^p-— 1.778  amperes. 

«         "        «  /.,.       u      similarly 0.889      " 

u  a          a    ^,          a  u  ^  ^  778       " 

Making  the  total 4.444     "  as  before. 

00.  Fig,  22  represents  the  case  of  a  low  tension  dyna- 
mo, E,  of  E.M.F.  seven  volts,  and  internal  resistance 
/'!,  0.03  ohms,  charging  two  storage  cells  in  series,  of  2.5 
volts  and  0.01  ohm  each,  through  two  leads  r2  and  r5  of 
0.1  ohm  each.  In  this  case  the  E.  M.  F.  of  the  storage 
cells  is  opposed  to  that  of  the  charging  battery  so  that 
the  total  E.  M.  F.,  E—  e  =  7  —  5  =  2  volts,  and  the 
total  resistance  ^  +  r2  +  ^  +  n  +  ^  —  0-25  ohm, 

E—  e          2 
making  the  current  /  =  ~~~R —  =  rT25  =  °  amperes. 


Here  the  drop  in  the  dynamo  armature  due  to  its 
resistance  will  be  E  =.  I  R  =  8  X  0.03  =  0.24  volt, 
leaving  a  p.  D.  at  dynamo  terminals  of  7  —  0.24  =  6.76 
volts.  The  drop  in  each  lead  will  be  8  X  0.10  =  0.8 
volts  or  1.6  volts,  collectively,  making  the  pressure  at 
storage  battery  terminals  5.16  volts,  and  the  apparent  drop 
in  each  cell  2.58  volts,  of  which  2.5  is  counter  E.  M.  F., 
and  0.08  p.  D.  due  to  I-R  drop. 

61.  Ohm's  law  for  the  current  strength  developed 
by  a  given  impressed  E.  M.  F.  in  a  circuft  of  given 
resistance,  is  not  true  unless  the  E.  M.  F.  impressed  on  the 
circuit  remains  constant  in  strength,  or  in  case  it  varies, 
allowance  is  made  for  its  variation.  In  the  case  of 
alternating  current  circuits,  where  the  variation  in  E.  M.  F. 
is  periodic  and  obeys  a  definite  law,  and  where  con- 
sequently the  current  strength  varies  periodically,  allow- 
ance has  to  be  made  for  the  effect  of  this  variation,  and 
the  current  strength  in  such  a  circuit,  is  not  generally 
the  simple  ratio  of  the  E.  M.  F.  to  the  resistance.  The 
necessary  modification  of  Ohm's  law  as  applied  to 
alternating  current  circuits,  will  be  explained  in  a  sub- 
sequent leaflet. 


55 
SYLLABUS. 

In  any  continuous  current  circuit,  the  current 
strength  varies  directly  with  the  E.  M.  F.  or  pressure; 
i.  e.,  if  the  pressure  be  doubled,  the  current  will  be 
doubled,  and  if  the  pressure  be  halved,  the  current  will 
be  halved. 

In  any  continuous  current  circuit  the  current  strength 
varies  inversely  as  the  resistance  ;  i.  e.,  if  the  resistance 
be  doubled  the  current  strength  will  be  halved,  and 
vice-versa.  Both  the  above  relations  are  expressed  in 
Ohm's  law  in  the  equation 


If  E,  the  total  E.  M.  F.  be  expressed  in  volts,  and  R  the 
total  resistance  in  ohms,  then  /,  the  current  strength  will 
be  expressed  in  amperes. 

The  resistance  in  any  circuit  can  usually  be  con- 
veniently divided  into  three  parts  ;  namely,  the  resistance 
of  the  source  ;  the  resistance  of  the  leads  or  conductors  ; 
and  the  resistance  of  the  receptive  device  ;  and  the 
formula  of  Ohm's  law  then  becomes 

i? 

1=  - 


+  fa  +  r* 

The  total  E.  M.  F.  in  a  circuit  is  equal  to  the  sum  of 
the  separate  E.  M.  F.'S  in  the  circuit  if  more  than  one 
E.  M.  F.  is  acting. 

The  fall  of  pressure  or  "  drop  "  in  a  conductor  carrying 
a  current,  is  the  E.  M.  F.  required  to  send  that  current 
through  the  conductor. 


(Hun's  law  for  current  strength  is  nut  applicable  to 
other  than  continuous  current  circuits,  unless  allowance- 
is  nuide  for  variations  in  i.  M.  i.  In  allernat  in^  current 
circuits  tlierelore,  where  the  i.  M.  r.  varies  peri(»(lic;ill\, 
thi>  la\\  re.|iiires  inodilicat  inn  tor  the  i  .  M.  i  .V  that  arc 
int  i-otliiced  into  the  circuit  l>\  the  \ariation  of  the 
current. 


l.:il»..r;il..r\    "I     lUuMim  \    K.-iin 

PhiltdelphU, 


|<  '"|M  "•    III  ,     1     •>(.     '".        'HI'      I'    I    '"     '  Mi    A  I       I'.  Mi.  IN  I'  Ml,     | 


WKKKI.N. 


Electrical    Engineering    Leaflets, 


Prof.  E.  J.  Houston,  Ph.  D. 

AND 

A.  E.  Kennelly,  F.  R.  A.  S. 


I  NT*  EF?  MEDIATE     GFtADE 

KI.KCVKMC    C 


,  ^2.  The  word  circuit,  litemllv  a  circle,  is  the  path 
which  MII  electric  current  traverses  when  it  lea\e.- 
the  positive  pole  of  Mil  electric  source,  or  hatlerv  of 
sources,  Mild  passes  through  or  iullueiices  the  various 
electro  receptive  or  I  I'M  lisla  t  i  11^  devices  placed  in  if-,  path, 
iv  enters  the  source  or  haltervat  its  ne^si.live  pole,  ;ind 
returns  to  its  starting  |>oint,  M(  the  positive  pole,  after 
(lowing  through  the  source.  Ill  actual  prMctice,  the. 
shape  ol  the  path  is  seldom  circular.  It  is  evident  HIM! 

all  conducting oircBita  consist  essentially  of  three  parts; 

namely  : 

(/.)   Of   the  source. 

(#.)   Of   the.  conductors. 

(.'/.)    Of    the  receptive  or  t  rMiishltin^   devices. 

'Flic  prime  ohject  of  all  elect  ric  circuits  is  to  conve\ 
an  electric  current,  produced  h\  Mil  electric  source,  to  more 
or  less  remote  places  where  the  electro  receptive  devices 
are  located. 


Till,   I.I-ECTRICAL  ENGINKER, 
303  "roadway,  Nrw  York,  N.  V. 

1  I    nl     i'  'I  .1  -    >' I  .'I. i-,,  ni.ill.  i   ..I    llic    Nc.w   V..ik,   N.   Y.,   I'osl  Ollirr,    June 


58 


63.  The  conditions  governing  the  current  strength 
in  any  circuit  will,  as  already  pointed  out,  be  de- 
termined in  accordance  with  Ohm's  law,  by  the  relations 
existing  between  the  electromotive  forces  and  the  resist- 
ances. 

The  resistance  of  a  circuit  will  necessarily  depend 
upon  the  separate  resistances  of  the  three  parts  already 
referred  to,  namely,  the  source,  the  leads,  and  the  recep- 
tive devices,  and,  since  the  useful  work  done  by  the 
various  receptive  devices  will  depend  upon  the  relation 
existing  between  their  resistance  and  the  resistance  of 
the  rest  of  the  circuit;  i.e.,  that  of  the  leads  and  sources, 
it  is  necessary  that  the  resistance  of  these  separate  parts 
be  properly  proportioned  in  order  to  obtain  the  desired 
efficiency. 

64.  In  order  to  obtain  the  best  relative  resistances 
adapted  to  the   conditions   of  different   cases,  a 

great  variety  of  conducting  paths  or  circuits  have  been 
devised.  These,  however,  may  readily  be  grouped  under 
four  leading  classes ;  namely  : 

(1.)  Series  circuits. 

(2.)  Multiple  circuits. 

(3.)  Multiple-series  circuits. 

(4.}  Series-multiple  circuits. 

65.  In  the  series  circuit  of  electro-receptive  devices 
all  the  current  passes  successively  through  each 

electro-receptive  device,  and  returns  to  the  source.  For 
example,  in  Fig.  23,  which  represents  a  Municipal  Series 
Incandescent  System,  the  current  leaving  the  dynamo,  E, 
at  its  positive  or  -f-  terminal,  passes  through  the  lamps  1, 
'2,  3,  4,  5, 10,  and  returns  .to  the  dynamo  at  its  negative  or 


59 


-  terminal.  Since  in  such  a  circuit,  the  extinguishment 
of  any  single  lamp  would  break  or  open  the  circuit,  and 
thus  render  all  the  other  electro-receptive  devices  in- 
operative, some  form  of  safety  device  is  always  em- 
ployed automatically  to  short-circuit  any  lamp  which 
may  become  faulty,  and  thus  permit  the  current  to  con- 
tinue to  pass  through  the  other  devices. 

W5.       A  series  circuit  of  electro-receptive  or  translat- 
ing devices  is  generally  employed  in  the  case  of  arc 
lamps,  and  of  most  telegraphic  and  telephonic  apparatus. 
The  resistance  of  a  series  circuit  is  equal  to  the  sum  of 
the  separate  resistances ;  consequently,  in  all  such  cir- 


PIG.  23. — "MUNICIPAL"  SERIES  CIRCUIT  OF  21  INCANDESCENT  LAMPS. 

cuits,  as  additional  translating  devices  are  introduced  or 
removed  from  the  circuit,  some  arrangement  must  be 
provided  in  the  source,  which  will  vary  its  electromotive 
force,  and  so  ensure  a  proper  working  of  the  electro-re- 
ceptive devices  by  causing  the  current  which  passes 
through  them  to  remain  constant. 

The  series  circuit  as  employed  for  arc  lamps  is,  for 
this  reason,  frequently  called  a  constant  current  circuit. 
A  constant  current  circuit  of  variable  resistance  must 
necessarily  be  a  circuit  of  variable  E.  M.  F. 

r>7.       Electric  sources  may  also  be  connected  in  series; 
for  example,  in  Fig.  24,  which  shows  two  dyna- 
mos, K!  and  E2,  connected  in  series.     Here  the  positive 


brush  of  the  dynamo,  E2,  is  connected  with  the  negative 
brush  of  the  dynamo,  EI?  and  their  free  brushes  are  con- 
nected respectively  to  the  negative  and  positive  leads. 
In  the  case  of  series  connection  of  electric  sources,  the 
electromotive  force  of  the  single  source  so  provided,  is, 
of  course,  equal  to  the  sum  of  the  electromotive  forces 
of  the  separate  sources.  Dynamos  are  so  coupled  when 
the  electro-receptive  devices  they  are  intended  to  operate, 
require  a  greater  electromotive  force  than  that  whicli 
either  dvnamo  alone  can  furnish. 


Etec.  Engineer 

FIG.  24. — SERIES-MULTIPLE  ARRANGEMENT  OF  LAMPS  AND  SERIES 
ARRANGEMENT  OF  DYNAMOS. 

68.  In  the  multiple  circuit  of  electro-receptive  de- 
vices, the  separate  electro-receptive  devices  have 
all  their  positive  terminals  connected  to  a  single  positive 
conductor  or  lead,  and  all  their  negative  terminals  simi- 
larly connected  to  a  single  negative  conductor  or  lead. 
The  current  from  the  source  passes  from  the  positive  lead 
through  as  many  separate  branches,  or  derived  circuits,  as 
there  are  conducting  paths  offered  to  it,  and,  after  passing 
through  the  receptive  devices,  returns  to  the  dynamo  at 
the  negative  lead.  Thus,  in  Fig.  25,  the  dynamo  D,  has  it 
positive  brush  or  terminal  connected  to  the  positive  lead, 
and  its  negative  brush  or  terminal  connected  to  the 
negative  lead.  The  separate  receptive  devices,  in  this 
case  a  group  of  lamps,  are  connected  as  shown,  each 


61 


with  one  of  their  terminals  to  the  positive  lead,  and  the 
other  to  the  negative  lead.  The  current  leaving  the 
dynamo,  branches,  as  shown  by  the  arrows,  and,  after 
passing  through  the  receptive  devices,  returns  to  the 
dynamo. 

69.  Since  in  the  multiple  circuit,  the  resistance  of 
the  entire  circuit  decreases  with  every  new  recep- 
tive device  added  in  multiple,  it  is  evident  that  the  cur- 
rent strength  in  the  entire  circuit  will  vary  with  the 
number  of  separate  receptive  devices;  but,  since  neg- 
lecting the  drop  in  the  leads,  the  lamps  are  all  subjected 
to  the  same  difference  of  potential,  it  is  evident  that  this 
circuit  will  have  between  its  leads  a  constant  difference 


INCANDESCENT  LAMPS  IN  PARALLEL 


f     «-    hi 


2  AMP.          1>£AMP.          1  AMP.         JSAMP. 

Multiple  Arc  Circuit 

FIG.  25. 

of  potential,  or  electrical  pressure,  depending  upon  the 
difference  of  potential  furnished  by  the  dynamo,  at  its 
brushes.  Such  a  circuit  is,  therefore-,  frequently  called 
a  constant  potential  circuit.  A  constant  potential  cii*- 
cuit  is  one  in  which  the  current  strength  in  the  circuit 
"necessarily  varies  with  the  number  of  devices  operated. 
The  electro-receptive  devices,  however,  since  their  resis- 
tances are  all  equal,  will  be  each  traversed  by  a  constant 
current,  because  they  are  acted  on  by  a  constant  electro- 
motive force. 

TO.       Electric  sources  may  be  connected  in  parallel. 
Thus,  Fig.  26,  shows  four  dynamos  of  equal  elec- 
tromotive forces,  such  as  would  be  employed  in  supply- 


62 


ing  incandescent  lamps  connected  in  multiple.  Here  all 
the  dynamos  have  their  positive  brushes  connected  to  a 
single  positive  lead,  or  bus-bar,  A  B,  and  all  their  nega- 
tive terminals  similarly  connected  to  a  single  negative 
lead,  or  bits-bar,  c  D.  The  electromotive  force  of  the 
combination  is  the  same  as  that  of  a  single  dynamo,  and 
the  resistance  of  the  combination,  much  less  than  that  of 
a  single  dynamo,  as  would  follow  from  the  rule  already 
stated  for  determining  resistances  in  parallel. 

A  bus-bar — an  abbreviation  of  omnibus  bar — receives, 
as  its  name  indicates,  the  total  current  supplied  by  two 
or  more  dynamos. 


C  .      .    .        D 

FIG.  26. — MULTIPLE  CONNECTION  OF  DYNAMOS. 

The  total  current  furnished  to  the  bus-bars  may,  there- 
fore, be  much  greater  than  in  the  case  of  a  single  dyna- 
mo, and,  when  all  the  dynamos  are  exactly  equal,  will  be 
just  four  times  as  great  when  each  dynamo  is  operated 
at  full  load. 

71.  In  the  multiple-series  circuit,  a  number  of 
separate  electro-receptive  devices  are  connected  in 
separate  groups  in  series,  and  these  groups  subsequently 
connected  in  parallel. 

Fig.  27,  shows  nine  plating  baths  connected  in  multi- 
ple-series. Here  the  baths  are  coupled  in  separate  series 
groups  of  three  each,  and  these  groups  subsequently  con- 


nected  in  multiple.  This  arrangement  is,  however, 
almost  entirely  limited  to  the  case  of  voltaic  cells,  and  is 
adopted  where  it  is  necessary  to  obtain  such  relations 
between  the  current  strength  and  the  electromotive  force 
as  may  be  required  for  the  best  operation  of  receptive 
devices  connected  in  the  circuit. 

72.       In  the  series-multiple  circuit,  a  number  of  sepa- 
rate   electro-receptive    devices   are  connected  in 
separate  groups  in  multiple,  and  these  groups  subsequently 
connected  in  series.     This  arrangement  is  employed  in 
the  case  of  the  three-wire  system  for  incandescent  lamps, 


?  ?  i1 

-L  e  6 


J;lvc. Engineer 

FIG.  27. — ARRANGEMENT  OF  ELECTROPLATING  BATHS  IN  MULTIPLE- 
SERIES. 

where,  as  indicated  in  Fig.  24,  the  lamps  are  connected 
in  separate  groups  in  multiple,  and  these  groups  sub- 
sequently connected  in  series.  In  order  properly  to 
maintain  the  electrical  pressure  at  the  terminals  H  B,  and 
H  D,  of  the  two  groups  of  lamps  when  the  number  of 
lamps  in  each  group  may  vary,  a  third  or  neutral  wire 
<;  H,  is  carried  from  the  point  H,  to  the  common  connect- 
ion G,  of  the  two  dynamos,  and  the  current  through  this 
wire  automatically  tends  to  equalize  the  pressure  on  each 
side  of  the  system. 


SYLLABUS.  * 

All  circuits  may  be  divided  into  four  main  classes; 
viz:  series,  multiple,  multiple-series,  and  series-multiple. 

The  resistance  of  electric  devices  or  sources  connected 
in  series  is  tlie  sum  of  their  separate  resistances ;  the 
electromotive  force  of  series-connected  sources  is  the  sum 
of  their  separate  electromotive  forces. 

In  the  series  circuit,  the  current  strength  is  constant 
throughout  the  circuit :  a  series  circuit  is,  therefore,  some, 
times  called  a  constant  current  circuit. 

In  a  multiple  circuit,  the  conductance  of  devices  or 
sources  connected  in  parallel  is  the  sum  of  their  separate 
conductances. 

In  the  multiple  circuit  the  potential  difference,  dis- 
regarding drop  in  the  leads,  is  constant.  This  circuit  is, 
therefore,  sometimes  called  a  constant  potential  circuit. 

Yoltaic  cells  are  sometimes  connected  in  multiple- 
series  or  in  series-multiple. 

Laboratory  of  Houston  &  Kennelly, 
Philadelphia. 


tCopyright,  1894,  by  TUB  ELFXTRICAL  ENGINEER.! 
WEEKLY. 

No.  9.  AUOTIST  11, 1894. 

Electrical   Engineering   Leaflets, 


Prof.  E.  J.  Houston,  Ph.  D. 

AND 

A.  E.  Kennelly,  F.  R.  A.  S. 


INTERMEDIATE     GRADE. 

VOLTAIC  CELL, 


73.  We  have  already  seen  that  the  origin  of  the  E. 
M.  F.  produced  by  the  friction  of  unlike  bodies  is 
to  be  traced  to  the  contact  of  dissimilar  surfaces.  Here 
the  energy  supplying  the  electricity  is  the  mechanical 
energy  required  to  produce  the  friction.  We  have  also 
seen  that  this  E.  M.  r.  can  be  made  to  produce  momentary 
currents.  When  the  contact  of  different  metallic  sub- 
stances is  produced  through  the  agency  of  a  liquid  cap- 
able of  conducting  electricity  and  of  being  decomposed 
by.  it,  such  contact  produces  an  E.  M.  F.  which  can  be  uti- 
lized for  the  production  of  a  steady  current. 

Y4.  A  device  for  the* ready  production  of  electro- 
motive force  by  the  contact  of  metallic  substances 
through  the  intervention  of  a  liquid  substance  is  called 
a  voltaic  cell.  In  all  cases  a  voltaic  cell  consists  of  two 
dissimilar  substances,  generally  metals,  and  an  exciting 
liquid,  called  the  electrolyte.  The  two  metallic  sub- 
stances form,  when  used  in  this  connection,  what  is  called 

Published  by 

THE  ELECTRICAL  ENGINEER, 
203  Broadway,  New  York,  N.  Y. 

[Entered  as  second-class  matter  at  the  New  York,  N.  Y.,  Post  Office,  June  14,  1894.] 


66 


a  voltaic  couple,  the  liquid  through  whose  intervention 
their  contact  is  continued  is  called  the  electrolyte,  and  each 
of  the  separate  metals  of  the  couple  is  called  an  element. 
Where  the  amount  of  current  required  for  use  is  not 
excessive,  a  voltaic  cell  is  one  of  the  most  convenient 
sources  of  electrical  energy.  It  is  named  in  honor  of 
Alexander  Yolta,  who  invented  it. 


FIG.  28.— SIMPLE  FORM  OF  VOL- 
TAIC CELL  ox  OPEN  CIRCUIT. 


El«c.Engineer 

FIG.  29. — SIMPLE  FORM  OF  VOL- 
TAIC CELL  ox  CLOSED  CIRCUIT. 


75.       When  as  in  Fig.  28,  two  plates  of  commercial 
zinc  and  copper  are  plunged  in  a  dilute  solution 
of  sulphuric  acid  in  water,  the  following  actions  take 
place ;  namely, 

(!.}  The  acid  acts  on  the  zinc  which  is  slowly  dissolved 
with  the  formation  of  zinc  sulphate,  ZnSO^  and  the  lib- 
eration of  hydrogen,  //>,  according  to  the  following 
chemical  equation, 

Zn       .#~  8O 


(2.)  The  hydrogen  is  liberated  entirely  at  the  surface 
of  the  zinc  plate,  where  the  action  occurs. 

(3.)  ~No  action  occurs  at  the  copper  plate. 

(4.)  The  chemical  action  on  the  zinc  plate  is  attended 
by  the  liberation  of  heat  which  raises  the  temperature 
of  the  liquid. 

When  now,  the  zinc  is  connected  outside  of  the  liquid 
by  means  of  conducting  wire,  as  shown  in  Fig.  29,  the 
phenomena  change  and  are  as  follows : — 

(1.)  The  zinc  is  attacked  as  before  with  the  formation 
of  zinc  sulphate,  though  not  necessarily  at  the  same  rate 
as  before. 

(2.)  The  hydrogen  is  now  liberated  almost  entirely  at 
the  surface  of  the  copper  plate. 

(3.)  The  heat  liberated  during  the  action  now  appears 
in  all  parts  of  the  circuit  and  is  not  confined  to  the  cell. 
That  is  to  say,  the  wire  becomes  warm. 

(4.)  An  electric  current  now  traverses  the  conducting 
circuit  as  may  be  shown  by  its  action  on  a  magnetic 
needle  suspended  either  near  the  wire  or  near  the  battery. 

76.  The  combination  of  parts  described  in  connection 
with  the  preceding  figure  constitutes  a  simple  form 
of  voltaic  cell.  The  source  of  energy  which  produces  the 
E.  M.  F.  is  clearly  to  be  traced  to  the  chemical  potential 
energy,  of  the  plates  and  electrolyte,  liberated  during 
the  chemical  combination  of  the  positive  element  \\rith 
the  negative  radical  of  the  electrolyte  ;  i.e.,  the  SO±  ion 
or  radical. 

The  chemical  equation  which  expresses  the  activity  of 
the  cell  is?  therefore, 

Before  action ;  Cn  -f-  JL  SO^  -f-  Zn. 
After  action  ;    Cn  +  IL  +  ZnS04. 


The  action,  therefore,  evidently  removes  an  atom  of 
zinc  for  every  molecule  of  JI2  /SOi  decomposed,  while 
the  hydrogen  liberated  tends  to  collect  on  the  surface  of 
the  copper  plate.  Since  this  hydrogen,  by  its  contact 
with  the  copper,  tends  to  produce  an  E.  M.  r.  directed 
opposite  to  that  of  the  cell,  its  presence  tends  to  decrease 
the  working  E.  M.  F.  and  various  devices  are  employed 
to  avoid  its  presence. 

Polarization  devices  in  practice  provide  for  either  pre- 
venting the  liberation  of  the  hydrogen  or  for  rapidly 
absorbing  it  after  formed.  Both  of  these  objects  are 
effected  by  surrounding  the  plate  by  some  suitable 
chemical  substance.  The  tendency  to  the  production  of 
a  counter-electromotive  force  by  the  presence  of  hydro- 
gen, is  called  the  polarization  of  the  cell,  and  the  sub- 
stance surrounding  the  negative  plate  for  the  purpose 
of  preventing  such  polarization  is  called  the  depolarizer. 

77.  Voltaic  cells  may  be  divided  into  the  following 
general  classes ;  namely, 

(1.)  Cells  without  depolarizers. 

(2.)  Cells  with  depolarizers. 

Those  of  the  first  class  are  generally  called  single-fluid 
cells,  and  in  them,  on.  closed  circuit,  polarization  is  apt  to 
prove  a  very  serious  defect.  The  best  cells  of  this  class 
employ  for  one  of  the  elements  a  carbon  plate.  Carbon, 
as  is  well  known,  possesses  in  a  marked  degree,  the  power 
of  dissolving  and  occluding  hydrogen  gas. 

When  an  exciting  liquid  like  chromic  acid  is  em- 
ployed, which,  besides  acting  on  the  zinc,  also  possesses 
the  power  of  combining  v/ith  and  dissolving  hydrogen, 
the  polarization,  which  would  otherwise  exist,  is  reduced. 


69 


Strictly  speaking,  then,  the  fluid  in  such  single-fluid  cells, 
acts  both  as  the  exciting  and  depolarizing  fluid. 

78.  Voltaic  cells  with  depolarizers  may  be  divided 
into  two  well  defined  classes ;  namely, 

(1.)  Cells  with  a  single  fluid  and  a  solid  depolarizer, 
and 

(&)  Cells  with  two  separate  fluids,  one  exciting,  and 
one  depolarizing,  with  a  porous  partition  between  them. 

In  the  simple  or  voltaic  cell  shown  in  Fig.  29,  the 
current  produced  on  the  closing  of  the  circuit  is  conven- 
tionally assumed  to  flow  in  the  direction  shown  by  the 
arrows ;  namely,  through  the  electrolyte  from  the  zinc 
plate  to  the  copper  plate,  and,  outside  the  electrolyte, 
from  the  copper  terminal,  through  the  conducting  path, 
to  the  zinc  terminal.  Since,  according  to  convention, 
that  pole  of  the  source  out  from  which  the  electricity 
flows  is  the  positive  pole,  and  that  pole  into  which  it 
flows,  the  negative  pole,  it  will  be  seen  that  the  positive 
terminal,  or  electrode,  is  the  terminal  connected  with  the 
copper  plate,  while  the  negative  terminal  or  electrode 
will  be  that  connected  with  the  zinc  plate.  Strictly 
speaking  this  convention  will  make  the  polarity  of  the 
plates,  where  covered  by  the  electrolyte,  exactly  opposite 
to  the  polarity  in  the  parts  above  the  liquid  ;  namely,  the 
zinc  plate  will  be  positive  in  the  liquid  and  the  copper 
plate  negative. 

It  is  evident  that  the  above  is  a  mere  convention,  that 
the  zinc  plate  cannot  be  both  positive  and  negative.  In- 
deed, if  tested  by  an  electrometer,  the  zinc  plate  can  be 
shown  to  be  negative  throughout  its  mass,  and  the  cop- 
per plate  can  be  similarly  shown  to  be  positive.  Still  it 


70 


is  convenient  to  refer  to  the  copper  plate  as  the  negative 
plate  because  the  current  enters  it  from  the  liquid,  and 
the  zinc  plate  as  the  positive  plate,  because  the  current 
issues  from  it  into  the  liquid  ;  and,  moreover,  since  these 
terms  are  sanctioned  by  general  usage  we  shall  employ 
them  in  future. 

Y9.  It  may  be  interesting,  in  connection  with  the 
preceding,  to  state  the  manner  in  which  the  con- 
vention as  to  the  direction  in  which  the  electric  current 
is  assumed  to  flow  (namely  from  the  positive  to  negative ) 
arose.  It  was  originally  arbitrarily  assumed  that  the 
character  of  the  electrification  produced  by  say,  catskin 
against  glass,  was  negative  on  the  catskin  and  positive  on 
the  glass.  The  glass  was,  therefore,  regarded  as  having 
a  positive  charge,  and  the  catskin,  a  negative  charge,  and, 
when  these  charges  neutralized  each  other,  it  was  as- 
sumed that  the  charge  passed  in  a  momentary  current  or 
discharge  from  the  glass  to  the  catskin ;  ?'.  <?.,  from  the 
positive  to  the  negative  substance.  When  at  a  later  date, 
Volta's  discovery,  showed  that  electric  charges  were  pro- 
duced by  the  contact  of  two  dissimilar  metals  through 
the  intervention  of  an  electrolyte,  it  was  found  by  the  use 
of  electrometers  that  the  charge  produced  at  the  copper 
plate  of  the  battery  was  of  the  same  sign  as  that  pro- 
duced on  glass  by  friction.  The  copper  pole  was,  there- 
fore, taken  as  the  positive  pole  of  the  cell,  and  this  being 
determined,  all  the  other  polarities  follow. 

80.       Since  hydrogen  invariably  appears  at  the  sur- 
face of  the  negative  plate,  a  depolarizing  substance 
should  be  placed  so  as  to  surround  the  negative  plate. 
Where  the  depolarizer  is  a  liquid,  the  use  of  a  porous 


cell  is  necessary,  in  order  to  prevent  the  admixture  of  the 
exciting  with  the  depolarizing  liquid. 

The  substance  employed  for  the  porous  cell  or  parti- 
tion generally  consists  of  unglazed  earthenware.  Since 
the  resistivity  of  the  substance  of  the  porous  cell  is  very 
high,  the  electric  current  produced  in  the  cell  passes 
almost  entirely  through  the  liquid,  following  the  minute 
pores  or  channels  in  its  substance,  and  the  resistance  of 
the  cell  is  necessarily  increased.  The  formation  of  crys- 
tals, or  the  collection  of  bubbles  of  gas  in  the  pores,  may 
also  still  further  increase  the  resistance.  Where  the  de- 
polarizer is  a  solid,  the  use  of  the  porous  cell  may  be  dis- 
pensed with  and  its  accompanying  disadvantages  avoided. 


SYLLABUS. 

In  a  voltaic  cell,  the  E.  M.  F.  is  produced  by  the  contact 
of  the  elements  of  the  voltaic  couple  with  the  electro- 
lyte. 

The  simple  chemical  solution  of  zinc  in  sulphuric  acid 
is  attended  with  a  local  evolution  of  heat.  The  electro- 
chemical solution  of  zinc  in  a  voltaic  cell  is  attended 
with  a  general  evolution  of  heat  together  with  the  pro- 
duction of  electric  current  through  the  circuit.  The 
counter-electromotive  force  of  a  cell  is  generally  due  to 
the  contact  of  hydrogen  with  the  negative  plate. 

Polarization  of  voltaic  cells  is  avoided  either  by  p re- 
venting  the  liberation  of  free  hydrogen,  or  its  removal 
by  combination,  if  liberated. 

Voltaic  cells  may  be  divided  into  two  classes,  those 
with  depolarizers,  and  those  without  depolarizers. 

Yoltaic  cells  with  depolarizers  are  of  two  classes, 
namely : 

Double  fluid  cells  containing  an  exciting  and  a  de- 
polarizing fluid ;  and  single  fluid  cells  containing  an  ex- 
citing fluid  writh  a  solid  depolarizer. 

Laboratory  of  Houston  &  Kennelly, 
Philadelphia. 


[Copyright,  1894,  by  THE  ELECTRICAL  ENGINFEK.] 
WEEKLY. 

No.  10.  AUGUST  18,  1894.       |Sripu™ 

Electrical   Engineering   Leaflets, 


-BY— 

Prof.  E.  J.  Houston,  Ph.  D. 

AND 

A.  E.  Kennelly,  F.  R.  A.  S. 


MEDIATE     CRADE. 

VOLTAIC  CELL, 


.  81.       The  objections  already  pointed  out  concerning 
the  use  of  single-fluid  cells,  have  prevented, -in  a 
great  measure,  their  commercial  introduction.     We  will, 
therefore,  describe  but  two  forms  of  single  fluid  bat- 
teries. 

Fig.  30  shows  a  form  of  plunge  battery,  commonly 
called  a  Grenet,  bichromate,  or  Poggendorf  cell,  although 
the  first  term  is  most  frequently  used.  This  cell  consists 
of  a  zinc-carbon  couple,  furnished  with  two  carbon  plates 
and  a  single  zinc  plate  placed  between  them.  In  the 
form  shown,  the  zinc  is  supported  on  a  slide  rod,  so  as  to 
enable  it  to  be  lifted  out  of  the  exciting  liquid  when  the 
battery  is  not  in  use.  The  exciting  liquid  consists  either 
of  a  solution  of  pure  chromic  acid,  or  of  a  mixture  of 
bichromate  of  potash  and  sulphuric  acid  in  water.  This 
mixture  forms  chromic  acid  and  a  salt  of  soda  and  is 
known  as  electropoion  fluid.  The  Grenet  cell  gives  an 
E.  M.  F.  of  1.9  volts  ;  as  its  resistance  is  about  0.2  ohm  it 

Published  by 

THE  ELECTRICAL  ENGINEER, 
203  Broadway,  New  York,  N.  Y. 

[Entered  as  second-class  matter  at  the  New  York,  N.  Y.,  Post  Office,  June  14,  1894.] 


• 


" 


nnrrrzE 


76 


Bunsen  cells  were  almost  entirely  used  for  the  produc- 
tion of  powerful  electric  currents,  but  they  are  now 
almost  entirely  superseded  by  cheaper  or  better  cells. 

85.  In  all  the  cells  thus  far  described  the  current 
strength  supplied  is  subject  to  considerable  varia- 
tion.    Daniell  was  the  first  to  produce,  in  1836,  a  voltaic 
cell   which    furnishes    a    practically    constant    current 
strength  on  closed  circuit,  provided  the  current  density 
in  the  cell  is  not  too  great.     The  Daniell  cell  is  a  zinc- 
copper  couple,  immersed,  respectively,  in  dilute  aqueous 
solutions  of  zinc  sulphate  and  concentrated  copper  sul- 
phate.    In  the  early  form  of  Daniell  cell,  the  copper 
sulphate  was  prevented  from  mixing  with  the  zinc  sul- 
phate by  being  placed  inside  a  porous  jar  of  unglazed 
earthenware.     Besides  the  objection  to  the  use  of  this 
cell  arising  from  the  high  resistance  already  referred  to, 
it  was  found  in  practice  that  the  copper,  instead  of  being 
deposited  on  the  surface  of  the  negative   plate,  was  de- 
posited irregularly  on  the  surface  of  the  porous  jar,  thus 
greatly  increasing   the   resistance   of    the   cell.     These 
objections  have  been  obviated  by  the  introduction  of  the 
Callaud  or  Gravity  type  of  Daniell  cell. 

86.  Fig.  33  shows  a  form  of  gravity  cell  as  generally 
constructed.     The  copper  element  is  placed  at  the 

bottom  of  the  glass  jar  and  is  provided  with  an  insulated 
wire  that  is  carried  out  at  the  top  of  the  jar.  The  zinc 
element,  in  the  form  of  an  open  wheel,  or  crow's  foot,  is 
supported  near  the  top  of  the  jar  as  shown.  To  charge 
the  cell,  enough  crystals  of  copper  sulphate  are  placed 
in  the  jar  to  entirely  cover  the  copper  plate,  and  the  jar 
is  filled  with  water  above  the  upper  surface  of  the  zinc 


77 

plate.  The  cell  is  then  placed  on  short  circuit  for 
about  twenty-four  hours,  in  order  to  form  enough  zinc  sul 
phate  in  solution  to  reduce  the  resistance  of  the  battery. 
The  constancy  of  the  Daniell  or  Callaud  cell  depends 
on  the  fact  that  polarization  is  almost  entirely  avoided, 
and,  instead  of  hydrogen  being  evolved  at  the  surface  of 
the  negative  plate,  there  is 'deposited  a  film  of  pure, 
highly  negative  copper.  Moreover,  the  exhaustion  of 
the  battery,  by  the  weakening  of  the  battery  solution,  is 
also  avoided  in  this  cell,  since,  as  fast  as  the  copper  sul- 


FIG.  33. — GRAVITY  DANIELL  CELL. 

phate  is  removed  from  solution  fresh  material  is  dissolved 
from  the  crystals  of  copper  sulphate  surrounding  the 
negative  plate,  and  the  solution  is  thus  kept  saturated. 
For  each  molecule  of  copper  sulphate  that  is  decomposed, 
one  atom  of  zinc  is  removed  from  the  zinc  plate  and  a 
molecule  of  zinc  sulphate  formed.  There  is  a  tendency, 
therefore,  for  the  zinc  sulphate  solution  to  become  con- 
centrated. This  is  readily  avoided  by  syphoning  off  a 
portion  of  the  zinc  sulphate  solution  and  replacing  it  by 
fresh  water.  When  most  of  the  crystals  of  copper  sulphate 


78 


have  been  dissolved,  it  is  only  necessary  to  throw  a  few 
handful s  of  fresh  crystals  into  the  cell.  After  the  cell 
has  been  in  use  for  some  time,  the  two  solutions  will  be 
observed  to  be  separated  from  each  other  by  a  sharply 
marked  line.  The  less  dense  solution  of  zinc  sulphate 
floats  on  the  denser  solution  of  copper  sulphate.  A 
Daniell  cell  gives  an  E.  M.  F.  of  about  1.072  volts. 

87.       Fig.  34  shows  a  form  of  single-fluid  cell,  called 

the  Leclanche,  with  a  solid  depolarizer  that  is  in 

very  extensive  commercial  use.     This  cell  consists  of  a 


FIG.  34. — LECLANCHE  CELL.  FIG.  35. — EDISON-LALANDE  CELL 
zinc-carbon  element  immersed  in  an  exciting  solution  of 
sal-ammoniac  in  water.  The  carbon  plate  is  placed  in  a 
porous  jar  and  is  surrounded  by  crushed  carbon  and 
black  oxide  of  manganese,  tightly  packed  in  the  porous 
jar.  The  top  of  the  porous  jar  is  then  sealed,  a  few 
openings  being  left  for  the  escape  of  gas.  The  carbon 
plate,  witli  its  porous  cell,  is  placed  inside  a  glass  jar,  as 
shown,  with  the  zinc  element  in  the  form  of  a  cylindrical 
rod  placed  beside  it,  A  Leclanche  cell  gives  an  E.  M.  F. 
of  about  1.47  volts.  This  cell  is  admirably  suited  for 
open-circuited  work.  It  readily  polarizes,  but  if  left  for 


sufficient  time  on  open  circuit,  recovers  its  normal  con- 
dition. It  is  extensively  employed  for  bell  and  signalling 
work. 

88.  Fig.  35  shows  an  Edison-Lalande  cell  of   300 
ampere-hours   capacity.      It   consists   of  a   zinc- 
copper  element  immersed  in  a  solution  of  caustic  soda  in 
water.     The  depolarizing  substance  is  a  solid ;  namely,  a 
block  of  compressed  copper  oxide,  placed  in  the  copper 
frame  of  the  negative  plate.     The  copper  plate  is  sus- 
pended between  two  parallel  plates  of  zinc,  and,  being 
separated  from  them  by  only  a  narrow  space,  and  the 
plates  being  comparatively  large,  the  internal  resistance 
of  the  cell  is  small,  being  in  the  case  of  the  size  of  cell 
shown  about     007  ohm. 

The  chemical  actions  which  take  place  are  as  follows ; 
viz.,  the  caustic  soda  is  decomposed  with  the  formation 
of  a  zincate  of  soda,  and  the  reduction  of  the  cupric  oxide 
at  the  negative  plate  to  metallic  copper.  When  the 
cell  is  exhausted,  the  zinc  plate  has  to  be  renewed,  and 
the  mass  of  metallic  copper  may  be  converted  in  a 
furnace  into  cupric  oxide. 

The  Edison-Lalande  cell  has  an  E.  M.  F.  of  about  f  volt. 

o 

It  has  practically  no  local  action  when  left  on  open  cir- 
cuit, and  possesses  the  advantage  of  being  able  to  furnish 
a  very  powerful  current,  and  also  to  remain  idle  for  long 
periods  of  time. 

89.  The  silver-chloride  cell  consists  of  a  zinc-silver 
couple  immersed  in  a  dilute  solution   of  sal-am- 
moniac in  water.     The  silver,  usually  in  the  form  of  a 
thin  wire  or  strip,  is  surrounded  by  a  bar  or  cylinder  of 
fused  silver  chloride. 


80 


When  charged,  this  cell  can  be  hermetically  sealed, 
and  in  such  form  can  be  readily  carried  about.  Owing 
to  the  cost  of  the  silver,  this  cell  is  only  employed  where 
comparatively  small  currents  are  required,  as  in  medical 
use,  or  for  testing  purposes.  A  silver  chloride  cell  has 
a  very  nearly  uniform  E.  M.  F.  of  1.03  volts. 

SYLLABUS. 

Nearly  all  practical  voltaic  cells  employ  depolarizers 
either  in  the  shape  of  liquids  or  solids.  Of  cells,  em- 
ploying liquid  depolarizers,  the  Grove,  the  Bunsen,  and 
the  Daniell  or  Callaud  are  the  most  important.  Of 
those  employing  solid  depolarizers,  the  Leclanche,  the 
Edison-Lalande,  and  the  silver-chloride  are  the  most  im- 
portant. 

The  Daniell,  or  Callaud  cell  is  very  well  adapted  to 
closed  circuit  work ;  the  Leclanche  cell  for  open  circuit 
work. 

The  Edison-Lalande  cell  is  suited  for  intermittent  and 
also  fdr  powerful  currents. 

The  silver-chloride  cell  furnishes  a  convenient  porta- 
ble form  of  battery  with  a  fairly  constant  E.  M.  F.,  but 
gives  only  a  comparatively  small  current. 

Laboratory  of  Houston  &  Kennelly, 
Philadelphia. 


[Copyright,  1894,  by  THE  ELECTRICAL  ENGINEER.] 
WEEKLY. 

~NY>    1 1  A  un  IT  QT  9^    1SQ4-         Price,    -    10  Cents. 

1  25,  1    M.        Subscription,  $3.00. 

Electrical   Engineering   Leaflets, 


Prof.  E.  J.  Houston,  Ph.  D. 

AND 

A.  E.  Kennelly,  F.  R.  A.  S. 


INTERMEDIATE     GRADE. 

VOLTAIC  CELL 


90.  No  matter  what  form  be  given  to  the  voltaic  cell, 
it  can  never  reasonably  be  expected  to  compete,  in 
point  of  economy,  with  a  well  designed  dynamo-electric 
machine,  where  any  considerable  output  of  electric  energy 
is  demanded.  The  source  of  energy  in  the  voltaic  cell  is 
the  chemical  potential  energy  of  the  plates  and  of  the 
electrolyte.  In  the  dynamo  electric  machine  the  energy 
liberated  from  coal,  burned  under  a  boiler,  is  eventually 
converted  into  electrical  energy  by  causing  the  conductors 
on  the  armature  to  cut  magnetic  flux  paths.  In  the  vol- 
taic cell  a  comparatively  expensive  metal,  zinc,  is  burned 
in  an  exciting  liquid,  and  the  output  of  the  cell,  is  neces- 
sarily much  higher  in  price  than  in  the  case  of  the  dynamo. 
Thus,  assume  the  cost  of  a  pound  of  zinc,  sufficiently  good 
to  be  employed  in  a  battery,  to  be  $0.07.  This  pound  of 
zinc,  as  we  have  seen,  would  be  consumed  by  a  delivery 
of  1,347,500  coulombs;  therefore,  if  the  E.  M.  F.  of  the 
cell  is  as  high  as  2  volts,  the  energy  delivered  by  the 

Published  by 

THEKELECTRICAL  ENGINEER, 
203  Broadway,  New  York   N.  Y. 

Entered  as  second-class  matter  at  the  New  York,  N.  Y.,  Post  Office,  June  14   1894.] 


82 


pound  of.  zinc  will  be  2,695,000  volt-coulombs  or  joules, 
and  dividing  this  by  the  number  of  seconds  in  an  hour, 
we  obtain  748.6  watt-hours,  =  0.7486  K.  w.  hours.  The 
price  of  a  K.  w.  hour  of  electric  energy  produced  from 
this  voltaic  cell  would,  therefore,  be  o-.^rib  =  9-35  cents 
in  zinc  consumed  alone,  regardless  of  the  cost  of  the 
electrolyte,  interest,  depreciation  and  attendance. 

On  the  other  hand,  it  is  well  known  that  large  engines 
require  about  1.8  Ibs.  of  coal  to  be  burned  under  their 
boilers  for  every  indicated  horse-power-hour  delivered  to 
the  dynamos,  so  that  with  coal  costing,  say,  $3.00  per  ton 
of  2,240  Ibs.,  the  cost  of  a  horse-power-hour  in  coal  is 
0.241  cent,  regardless  of  water,  oil,  waste,  attendance, 
interest  and  depreciation.  This  represents  0.323  cent 
per  indicated  K.  w.  hour,  and  with  dynamos  converting 
90  per  cent,  of  the  indicated  horse  power  into  electrical 
energy,  the  cost  of  coal  per  K.  w.  hour  is,  therefore  0.359 
cent.  It  will,  consequently,  be  seen  that  the  cost  of  a 
K.  w.  hour  produced  by  a  voltaic  cell,  in  zinc  consumed,  is 
about  26  times  the  cost  of  the  same  amount  of  energy 
(3600  joules)  produced  by  a  steam  dynamo  on  a  large  scale. 

91.  The  above  has  reference  only  to  the  case  where 
a  voltaic  battery  is  required  to  produce  a  large 
amount  of  electrical  energy.  In  the  case  of  the  stean: 
engine,  the  necessary  expense  for  attendance,  as  well  as  the 
inconveniences  attending  the  delivery  of  a  very  small 
amount  of  power,  renders  the  battery  a  very  convenient 
source  of  energy  for  such  small  powers  as  driving  sewing 
machines,  fans  or  other  similar  apparatus. 

The  amount  of  power  delivered  to  the  moving  air  by  a 
fan  of  nine  inches  in  diameter,  running  at  1,000  revolutions 


83 


per  minute,  is  about  three  watts,  increasing  very  nearly  as 
the  cube  of  the  velocity  up  to  a  critical  speed. 

The  amount  of  energy  absorbed  by  an  ordinary  sewing 
machine  making  about  480  stitches  per  minute,  is  approxi- 
mately 12  watts.  The  efficiency  of  the  small  motors  re- 
quired to  drive  the  fan  or  sewing  machine  being  usually 
no  more  than  0.5,  about  twice  the  mechanical  energy 
must  be  supplied  electrically  in  every  case. 

92.  A  ready  measure  of  the  amount  of  power  which 
a  cell  can  furnish,  neglecting  the  consequences  of 
polarization,  is  the  square  of  its  E.  M.  F.,  divided  by  its 
resistance.  This  may  be  called  the  electrical  capability 
of  the  cell  and  is  equal  to  the  number  of  watts  which 
would  be  expended  by  the  cell  if  placed  on  short  circuit, 
being  expressed  thus : 

P  =  ^ 

r 

It  is  evident,  therefore,  from  the  above  expression 
that  the  electrical  capability  of  a  cell  increases  as  the 
square  of  its  E.  M.  F.,  and  is  inversely  as  its  resistance.  In 
the  case  of  any  given  cell  the  E.  M.  F.  is  beyond  control. 
In  any  battery  the  E.  M.  F.  can  be  increased  by  adding  cells 
in  series.  The  internal  resistance  of  a  cell  can  be  decreased 
by  decreasing  the  distance  between  the  plates  or  by  in- 
creasing their  area.  The  latter  is  usually  the  most  prac- 
tical method.  It  will,  therefore,  be  seen  that  all  methods 
for  increasing  the  electrical  capability  of  a  voltaic  source 
are  practically  limited  to  coupling  a  number  of  cells 
together  so  as  to  increase  the  E.  M.  F.  or  to  decrease  the 
resistance,  or  both. 

The  electrical  capability  of  a  battery  of  N^  cells  is  JY 
times  the  electrical  capability  of  a  single  cell,  and  is  inde- 


pendent  of  the  grouping  of  the  cells,  provided  that  the 
cells  are  all  similar,  and  are  symmetrically  grouped. 
For  example,  if  a  cell  has  an  E.  M.  F.  of  2  volts,  and  an 
internal  resistance  of  0.25  ohm,  its  electrical  capability 

will  be  2  X  2  =  16  watts.     A  battery  of  24  such  cells 
U.^o 

would  have  16  X  24  =  384  watts.  For  if  arranged  in 
one  series  the  capability  of  the  battery  would  be 
/2  \/  24"\2 

^  ^  —  ^-  =  384  ;  or  if  arranged  in  2  series  of  12  each 
U.^o  X  2i^. 

in  multiple,    the    capability   of   the   battery    would   be 
(2  X  12)2 
25        -\    =    384,   and  so   on  for  any  symmetrical 


/0.25        -j£\ 


grouping  of  rows  and  series. 

93  When  a  given  small  amount  of  activity  is  to  be 
furnished  by  a  voltaic  battery,  the  first  considera- 
tion is,  of  course,  to  obtain  this  activity  with  a  maximum 
economy.  The  maximum  economy  may  be  either  the 
maximum  economy  of  operating  the  battery,  or  the 
maximum  economy  of  installing  it,  which,  of  course,  are 
entirely  different. 

The  maximum  economy  of  operation  is  obtained  when 
the  number  of  cells  is  such  that  the  materials  they  con- 
sume, together  with  the  interest,  depreciation  and  attend- 
ance, on  the  whole  plant,  is  a  minimum.  The  maximum 
economy  of  installation  is  obtained  when  the  number  of 
cells  employed  that  will  satisfactorily  perform  the  re- 
quired activity  is  a  minimum.  If  P,  be  the  amount  of 
this  activity  expressed  in  watts,  then  the  minimum  num- 
ber of  cells  is  4P,  divided  by  the  capability  of  each  cell. 

Or  the  number  of  cells,  N  =  —  —  .     In  other  words, 


the  maximum  economy  of  first  installation  is  reached 
when  the  capability  of  the  battery  is  four  times  the 
activity. 

Thus,  if  a  battery  were  required  to  operate  a  sewing 
machine  taking  12  watts,  through  a  motor  of  0.5  effi- 
ciency, requiring  24  watts,  and  if  each  cell  of  the  battery 
to  be  used  had  an  E.  M.  F.  of  1  volt  and  an  internal 
resistance  of  0.125  ohm,  its  capability  would  be  8  watts, 
and  the  minimum  number  of  cells  required  would  be 
4X24  =  12 

8 

This  number  of  cells  would  be  independent  of  the 
grouping  adopted,  for  the  same  results  would  be  ob- 
tained with  12  cells  in  a  single  series,  two  rows  of  six, 
three  rows  of  four,  four  rows  of  three,  six  rows  of  two, 
or  by  all  twelve  cells  in  parallel.  Practically,  however, 
the  arrangement  would  depend  on  the  winding  of  the 
motor.  If  the  motor  were  wound  for  six  volts,  then  all 
the  cells  would  be  connected  in  one  series.  The  E.  M.  F. 
of  the  battery  would  be  12  volts,  its  internal  resistance 

12   X   12 
1.5  ohms,  and  its  electrical  capability  ^ =  96 

1.5 

watts.  If  the  battery  were  placed  on  short-circuit  it 
would  therefore  do  work  through  its  own  resistance  at 
the  rate  of  96  watts,  which  is  four  times  the  external 
activity  or  output  required.  On  connecting  with  the 
motor,  the  current  delivered  at  the  required  output 
would  be  4  amperes.  The  drop  in  the  battery  would  be 
1R  —  4  X  1-5  =  6  volts,  or  half  the  E.  M.  F.,  and  the 
pressure  remaining  at  terminals  would  be  6  volts.  The 
external  activity  would  be  24  watts,  the  internal  activity 
24  watts,  the  total  activity  in  the  circuit  48  watts  or  half 

rgffZ — !— 55 

^OR-A. 
0V  THB 

IUHITBRSITT; 


86 


the  capability  of  the  battery,  and  the  efficiency  of  the 
battery  would  be  0.5. 

94.  Minimum  cost  in  battery  installation  satisfies  the 
following  six  conditions : 

(1.)  The  capability  of  the  battery  is  four  times  the  output. 

(2.)  The  capability  of  the  battery  is  twice  its  total 
activity. 

(3.)  The  internal  and  external  activities  are  equal. 

(^.)  The  pressure  at  battery  terminals  is  equal  to  the 
drop  in  the  battery. 

(5.)  The  efficiency  of  the  battery  is  0.5. 

(6.)  The  current  strength  through  each  cell  is  half 
that  which  it  would  deliver  on  short-circuit. 

95.  The  voltaic  cell  ranks  high  as  an  electric  source 
for    the    production    of  comparatively    constant 

E.  M.  F.'S.  Ordinarily  the  E.  M.  F.  of  a  voltaic  cell,  as  usu- 
ally constructed,  is  variable,  depending  as  it  does  upon  a 
number  of  circumstances  such  as  temperature,  strength 
of  solution,  purity  of  plates  and  exciting  liquid,  atmos- 
pheric pressure,  and  activity  of  the  circuit.  Yet  if 
certain  precautions  are  taken  in  the  preparation  and  use 
of  voltaic  cells,  they  can  be  made  to  produce  a  very 
uniform  E.  M.  F.  The  E.  M.  F.  of  a  dynamo  machine  can 
only  remain  uniform  when  the  speed  of  rotation  is 
maintained  constant  and  the  field  magnets  retain  a  con- 
stant strength,  conditions  which  are  frequently  difficult 
or  expensive  to  maintain,  when  only  a  small  amount  of 
power  is  desired.  On  the  contrary,  in  the  case  of  the 
voltaic  cell,  if  fairly  uniform  conditions  are  assured,  the 
value  of  the  E.  M.  F.  will  remain  very  nearly  constant. 
So  true  is  this  that  the  legal  value  of  the  unit  of  E.  M.  F., 


87 


the  volt,  lias  been  decided  as  being  a  definite  fraction  of 
that  furnished  by  a  particular  form  of  standard  cell 
known  as  a  Clark  standard,  at  a  definite  temperature, 
(y.J^-th  part  at  15°  C.) 

96.  The  following  fields  of  usefulness  exist  at  present 
for  the  voltaic  battery  : 

(1.)  As  a  limited  source  of  power.  This  we  have  seen 
is  limited  by  the  expense  of  materials  and  of  operation. 
The  limit  is  apparently  reached  in  practice  at  the  power 
required  to  drive  a  sewing  machine,  which  at  ordinary 
moderate  speeds,  as  already  mentioned,  is  about  twelve 
watts.  Allowing  an  efficiency  of  0.5  in  the  small  driving 
motor,  the  delivery  of  power  from  the  battery  becomes 
about  24  watts.  Beyond  this  amount  of  power,  the  ex- 
penses of  installing  and  maintaining  a  battery  is,  even  with 
the  best  existing  types,  generally  regarded  as  prohibitory. 

(#.)  For  signalling  purposes,  as  in  telegraphy,  tele- 
phony, annunciators,  etc.  Here  the  amount  of  work 
required  is  usually  very  small.  The  amount  of  energy 
delivered  by  a  battery  of  100  cells  to  a  telegraph  line  be- 
ing usually  about  three  watts.  In  all  large  telegraph 
stations,  dynamos,  or  storage  cells  charged  by  dynamos, 
are  being  generally  employed. 

(&)  For  testing  purposes,  as,  for  example,  in  furnishing 
a  uniform  E.  M.  F. 

(4>)  For  electroplating ;  although  here  also,  on  all  but 
the  smallest  scale  of  operation,  the  dynamo  has  come 
into  use. 

(5.)  For  electro-therapeutic  purposes^  owing  to  the 
portability  of  a  voltaic  battery. 

It  will  be  seen,  therefore,  that  the  principal  uses  for 
voltaic  cells  are  for  testing,  and  for  domestic  service 


88 


where  a  current  from  dynamos  cannot  readily  be  obtained. 
This  statement  refers  to  the  existing  batteries  which  burn 
zinc.  Should  it  become  practically  possible  to  consume 
carbon  in  a  commercial  voltaic  battery  and  obtain  an 
output  approaching  the  theoretical  amount,  it  might 
readily  become  much  cheaper  to  produce  electrical  energy 
from  batteries  than  from  dynamos,  and  the  use  of  the 
steam  engine,  as  the  prime  source  of  electric  power, 
might  be  superseded. 

SYLLABUS. 

The  source  of  energy  in  a  voltaic  cell  is  the  chemical 
potential  energy  of  the  plates  of  the  electrolyte. 

Voltaic  batteries,  as  at  present  constructed,  can  never 
compete  with  steam-driven  dynamos  for  the  delivery  of 
large  electric  currents.  For  small  powers,  however,  vol- 
taic batteries  possess  many  advantages  over  dynamos. 

The  electrical  capability  of  a  cell,  or  the  amount  of 
power  the  cell  is  capable  of  furnishing  when  placed  on 
short  circuit,  is  equal  to  the  square  of  the  E.  M.  F.  of  the 
cell  divided  by  its  resistance. 

The  electric  cell,  when  suitably  constructed,  ranks 
high  as  a  source  of  extremely  uniform  E.  M.  F.  In  this 
respect  it  far  surpasses  the  dynamo. 

The  legal  value  of  the  volt,  the  unit  of  E.  M.  F.,  is 
taken  as  that  furnished  by  a  particular  form  of  Clark 
standard  cell,  at  a  fixed  temperature. 

The  following  are  the  principal  fields  of  usefulness  for 
the  voltaic  cell,  viz.: 

(1.)  As  a  limited  source  of  power.  (2.)  For  signalling 
purposes.  (3.)  For  testing  purposes.  (4.)  For  electro- 
plating. (£.)  For  electro-therapeutics. 

Laboratory  of  Houston  &  Keimeily. 
Philadelphia. 


[Copyright,  1894,  by  THE  EI.FCTKICAI.  ENGINEER.] 
WEEKLY. 

No.  12.  SEPTEMBER  1,  1804. 

Electrical   Engineering   Leaflets, 


Prof.  E.  J.  Houston,  Ph.  D. 

AND 

A.  E.  Kennelly,  F.  R.  A.  S. 


INTERMEDIATE     Gf^ADE. 

IV!  as  tie  to  motive  Korce 


97.  Surrounding  every  magnet  there  is  a  region  of 
•    magnetic  influence,  technically  known  as  the  mag- 
netic field.     The  region  is  permeated  with  what  are  fre- 
quently called  lines  of  magnetic  force,  but  which  may  be 
more  accurately  described  as  magnetic  flux-paths. 

Magnetic  flux  possesses  the  following  properties, 
namely :  (1.)  A  bar  of  iron  when  introduced  into  the  flux 
becomes  magnetized.  (2.)  A  freely  suspended  magnetic 
needle  brought  into  the  flux,  comes  to  rest  in  a  definite 
position.  (3.)  An  electric  conductor  moved  across  the 
flux  paths  has  an  electromotive  force  developed  in  it. 

The  exact  nature  of  magnetic  flux  is  not  understood, 
but  it  appears  to  be  attended  by  a  stress  in  the  ether. 

98.  A  convention  is  employed  as  to  the  assumed 
direction  of  the  magnetic  flux,  similar  to  that  em- 
ployed in  the  case  of  electric  flux ;  viz.,  the  magnetic 
flux  is  assumed  to  issue  from  the  magnet,  as  shown  in 
Fig.  36,  at  its  positive  or  north,  i.  e.,  north-seeking  pole  N, 


Published  by 
THE   ELECTRICAL  ENGINEER, 

203  Broadway,  New  York   N.  Y. 

[Entered  as  second-class  matter  at  the  New  York,  N.  Y.,  Post  Office,  June  14   1894.] 


90 


(that which  tends  to  point  northwards  if  the  needle  be  free 
to  move),  and,  after  passing  through  the  region  around  the 


FIG.  36. — DIAGRAM  OF  ASSUMED  DIRECTION  OF  FLUX-PATHS  ix  A 
MAGNETIC  CIRCUIT. 

magnet,  t;>  re-enter  it  at  its  negative  or  south-seeking 
pole,  s,  thus  corresponding  with  the  direction  of  the 
electric  flux,  which  is  assumed  to  leave  a-n  electric  source 


FIG.  37. — PERMANENT  MAGNET  AND  FLUX-PATHS  SURROUNDING  IT, 
AS  INDICATED  BY  IRON  FILINGS  ON  PLATE  LAID  FLAT  ON  MAGNET. 

at  its  positive  pole  and  re-enter  it  after  having  passed 
through  the  circuit  at  its  negative  pole. 


91 


99.  Figs.  37  and  38  represent  the  assumed  direction 
of  the  magnetic  flux  in  the  case  of  a  magnet  of  the 
form  shown,  placed  in  Fig.  37,  with  its  greatest  length  hori- 
zontal to  the  plate,  and  in  Fig.  38,  with  its  greatest  length 
vertical  to  the  plate.  A  careful  inspection  of  these 
figures  will  show  that  the  poles  are  not  by  any  means 
concentrated  at  points  situated  at  the  extremities  of  the 
har,  but  are  distributed  over  a  considerable  area. 


FIG.  38. — FLUX-PATHS  SURROUNDING  MAGNET  POLES  AS  INDICATED  BY 
IRON  FILINGS  ON  PLATE  LAID  UPON  THE  POLAR  EXTREMITIES. 

Magnetic  figures  may  be  obtained  by  suitably  support- 
ing a  sheet  of  paper  over  a  magnet,  sprinkling  iron  filings 
upon  the  paper,  and  then  gently  tapping  it  so  as  to  enable 
the  filings  to  arrange  themselves  under  the  influence  of 
the  magnetic  forces. 

100.     The  preceding  figures  show  the  flux-paths  only 
in  the  immediate  vicinity  of  the  magnet,  being  lim- 
ited by  the  size  of  the  sheet  of  paper  employed.     In  reality 
the  magnetic  flux  exists  for  indefinitely  great  distances 


92 


around  the  magnet,  but  at  distances  exceeding  a  few 
inches  becomes  so  weakened  that  its  detection  requires 
the  use  of  comparatively  delicate  apparatus. 

The  magnetic  flux-paths,  around  any  bar  magnet, 
as  they  may  be  traced  either  by  iron  filings,  or  by  an 
exploring  'magnetic  needle,  (i.e.,  a  suspended  magnetic 
needle  which  assumes  the  direction  of  the  flux  at  the 
point  it  occupies)  show  that  the  flux  paths  coincide  with 
the  stream-lines  which  would  be  produced  by  a  tube 
filled  with  and  surrounded  by  an  incompressible  liquid, 
such  as  water,  if  a  force  pump  within  the  tube,  drove 
the  liquid  out  at  one  end  of  the  tube  and  sucked  it  in  at 
the  other  end.  In  the  case  of  a  magnet,  the  magnetic 
stream-lines,  as  already  remarked,  are  assumed  to  pass  out 
from  the  north  pole  and  re-enter  at  the  south  pole.  The 
force  which  causes  this  flux,  corresponding  to  the  force 
driving  the  pump  producing  the  liquid  flux,  is  called  the 
magnetomotive  force,  usually  abbreviated  M.  M.  F.  The 
M.  M.  F.,  in  the  case  of  the  magnetic  circuit,  corresponds 
to  the  E.  M.  F.  in  the  case  of  an  electric  circuit. 

We  have  seen  that  in  the  electric  circuit,  no  flux,  i.e., 
no  current  can  exist  without  the  establishment  of  an 
E.  M.  F.  in  the  circuit.  Similarly,  in  the  magnetic  cir- 
cuit, no  flux  can  exist  without  the  establishment  of  a 
M.  M.  F.  in  the  circuit. 

The  unit  of  magnetomotive  force  is  called  the  gilbert, 
from  Dr.  Gilbert  of  Colchester,  a  famous  early  authority 
on  magnetism,  (1600  A.D  ) 

101.     There   are   two   distinct   varieties  of   M.  M.  F.: 

namely,  the  permanent,  or  that  naturally  existing 

in  certain  kinds  of  matter,  notably  in  iron,  nickel,  cobalt ; 

and  the  transient,  or  that  produced  in  the  neighborhood 


93 


of  a  conductor  by  the  passage  through  it  of  an  electric 
current.  When  an  electric  current  circulates  through  a 
conductor,  a  certain  distribution  of  flux  is  produced  in 
the  region  surrounding  the  conductor.  If,  however,  this 
region  is  occupied  by  iron,  the  amount  of  flux  produced 
is  enormously  increased,  and  the  only  explanation  con- 
sistent with  the  facts  is  that  there  is  a  source  of  M.  M.  F. 
in  the  magnetized  iron  as  well  as  in  the  electric  current ; 


FIG.  39. — SECTION  OF  A  COMMON  TYPE  OF  DYNAMO  WITH  MAGNETIC 
CIRCUIT  INDICATED. 

for,  if  the  iron  be  hard,  its  magnetic  condition  will  in  a 
great  measure  persist  after  the  magnetizing  current  has 
ceased  to  flow,  in  which  case  the  iron  must  be  regarded  as 
the  seat  of  a  permanent  M.  M.  F. 

102.     In   nearly  all   practical   magnetic   circuits,  the 
magnetic  flux  passes,  for  the  greater  part  of  its 
path,  through  iron.     Thus  in  Fig.  39  is  shown  an  ordi- 
nary bi-polar  dynamo  in  which  the  magnetic  circuit  is 


94 


indicated  by  the  dotted  lines.  Here  the  path  through 
the  field  magnets  is  entirely  of  iron ;  through  the  arma- 
ture largely  of  iron,  and  through  the  interpolar  spaces 
between  the  armature  and  the  surrounding  pole  faces, 
A  A,  through  air. 

In  the  multipolar  dynamo  shown  in  Fig.  40,  the  mag- 
netic circuits  passing  through  each  pole,  divide,  passing 
as  before  through  circuits,  indicated  by  the  dotted  lines, 


FIG.  40. — DIAGRAMMATIC  SECTION  OF  A  SEXTIPOLAR  DYNAMO,  SHOWING 
THE  MAGNETIC  CIRCUITS  AND  GENERAL  ARRANGEMENT  OF  FLUX 
DISTRIBUTION  UNDER  THE  EXCITING-COIL  M.  M.  F.'S. 

lying  mainly  through  iron,  as  at  F,  but  partially  through 
air,  as  at  B. 

The  M.  M.  F.  which  drives  the  magnetic  flux  through 
any  circuit  is  dependent  on  two  factors;  namely,  the 
current  strength  passing  through  the  magnetizing  coils, 
and  the  number  of  turns  of  wire  in  these  coils.  This 
product  is  generally  expressed  in  ampere-turns. 


95 


103.  The  unit  of  M.  M.  F.  is  the  gilbert.     It  is  the 
M.  jyi.  F.  produced  by  — or  approximately  0.7958 

T:  7[ 

ampere-turn.  It  is  only  necessary,  therefore,  to  multiply 
the  number  of  ampere-turns  on  the  field  magnets  of  a 

dynamo  machine  by  —  1.257,  to  obtain  then.  M.  F. 

0.7958 

expressed  in  gilberts.  For  example,  a  particular  10  K.  w. 
dynamo  of  the  bi-polar  type  has  two  field  coils,  one  011 
eaclx  magnet.  The  total  number  of  turns  on  these  two 
coils  is  2,100,  and  the  current,  which  circulates  through 
these  coils  at  full  load,  is  2  amperes.  The  M.  M.  F.  in 
ampere-turns  is,  therefore,  4,200,  and  in  gilberts,  5,279. 

104.  "While   magnetomotive  forces    may    be   conve- 
niently and  accurately  expressed  in  ampere-turns, 

the  c.  G.  s.  system  of  International  measures  requires  that 
the  unit  of  M.  M.  F.  should  differ  by  a  numerical  factor. 
In  dealing  with  M.  M.  F.'S,  it  is  commonly  convenient  to 
express  their  values  in  ampere-turns,  but  for  purposes  of 
computation,  and  for  simplicity  of  reasoning,  it  is  usually 
advantageous  to  employ  the  more  fundamental  and  scien- 
tific unit,  the  gilbert. 

105.  Magnetomotive  force,  like  electromotive  force, 
possesses  direction.      That  is,  several  M.  M.  F.'S 

may  oppose  or  aid  one  another,  the  resultant  M.  M.  F. 
being  their  geometrical  sum,  precisely  like  the  case  of 
various  E.  M.  F.'S  acting  in  an  electric  circuit.  Thus  the 
M.  M.  F.'S  produced  in  the  magnetic  circuit  of  a  dynamo, 
by  two  separate  magnetizing  coils,  as  shown  in  Fig.  39, 
will  be  additive,  if  the  exciting  coils  are  magnetized  by 
currents  in  the  same  direction,  and  sub  tractive  if  the  coils 
are  magnetized  in  opposite  directions. 


96 


The  M.  M.  F.  produced  by  a  current  of  100  amperes, 
passing  through  a  single  loop  of  conducting  wire,  would 
be  100  ampere-turns,  or  125.7  gilberts,  whether  that  turn 
of  wire  were  alone  or  whether  it  were  associated  with 
other  turns  of  wire  in  a  coil,  and  whether  it  surrounded 
iron  or  not ;  but  the  flux,  which  that  M.  M.  F.  would  pro- 
duce, would  vary  very  greatly  in  these  different  cases. 

SYLLABUS. 

A  magnetic  field,  or  a  region  permeated  by  magnetic 
flux,  accompanies  every  magnet  or  every  conductor  con- 
veying an  electric  current. 

A  magnetic  field  produces,  or  is  accompanied  by,  a 
stress  in  the  ether,  which  may  manifest  its  presence  in  a 
variety  of  ways. 

The  density  of  the  magnetic  flux  in  any  field  is  greater 
near  the  magnet  than  at  distances  from  the  magnet,  and 
is  usually  at  its  maximum  value  in  the  neighborhood  of 
the  magnetic  poles. 

The  unit  of  M.  M.  F.  is  termed  the  gilbert,  and  is  equal 
to  the  M.  M.  F.  produced  by  0.7958  ampere-turn. 

All  magnetic  flux,  i.e.,  all  magnetism,  is  produced  by 
M.  M.  F.,  just  as  all  electric  flux  or  current  is  produced  by 
E.  M.  F. 

M.  M.  F.'S,  like  E.  M.  F.'S,  possess  direction,  so  that  several 
M.  M.  F.'S  may  oppose  or  aid  one  an  other ;  that  is,  their 
general  effect  is  either  subtractive  or  additive. 

Laboratory  of  Houston  &  Kennelly, 
Philadelphia. 


[Copyright,  1894,  by  THE  ELECTRICAL  ENGINEER.! 


WEEKLY. 


No.  13.  SEPTEMBER  8,  1894. 


Electrical   Engineering  Leaflets, 


—  BY— 

Prof.  E.  J.  Houston,  Ph.  D. 

AND 

A.  E.  Kennelly,  F.  R.  A.  S. 


IWTTEI^JYIEDIATE     GJ*AI>E. 

Magnetic  Reluctance 


106.  The  magnetic  flux  produced  in  any  magnetic 
circuit  by  a  given  M.  M.  F.  depends  upon  the  mag- 
netic resistance  of  the  circuit.  In  this  respect  magnetic 
resistance  is  similar  to  the  resistance  which  an  electric 
circuit  offers  to  the  passage  of  an  electric  flux  under  the 
influence  of  a  given  E.  M.  F.  Magnetic  resistance  is  called 
reluctance.  In  order  to  increase  the  magnetic  flux  under 
given  M.  M.  F.  it  is  only  necessary  to  decrease  the  reluct- 
ance of  the  circuit. 

The  following  differences  exist  between  magnetic  re- 
luctance and  electric  resistance ;  viz., 

(1.)  Unlike  the  resistance  in  an  electric  circuit,  the 
reluctance  of  masses  of  similar  dimensions  of  nearly  all 
materials,  except  iron  and  the  magnetic  metals,  is  practi- 
cally the  same  as  that  of  air. 

(&)  The  electric  flux  can  be  confined  to  a  definite  path, 
usually  a  wire,  while  the  magnetic  flux,  in  general,  can- 
not. The  reason  is  that  an  electric  conductor  can  be 


Published  by 

THE   ELECTRICAL  ENGINEER, 
203  Broadway,  New  Vork   N.  Y. 

[Entered  as  second-class  matter  at  the  New  York,  N.  Y.,  Post  Office,  June  14,  1894.] 


98 


readily  insulated,  whereas  there  is  no  known  insulator 
for  the  .magnetic  flux.  The  magnetic  flux  which  pro- 
ceeds from  the  north-seeking  pole  of  a  magnetic  source 
passes  through  numerous  diverging  paths,  re-entering 
the  magnet  at  its  south-seeking  pole. 

(3.)  In  the  case  of  an  electric  circuit,  where  a  long 
single  wire  sustains  a  steady  current,  the  current 
density  is  the  same  at  all  points  in  any  cross-section  of 
the  wire ;  in  the  case  of  a  magnetic  circuit,  the  flux 
density,  in  general,  varies  at  different  points  in  the  cross- 
section  of  the  circuit,  and  decreases  as  we  recede  from 
the  poles. 

107.  As  the  specific  resistance  of  a  conductor  is  best 
defined  under  the  term  resistivity;  namely,  the  resis- 
tance offered  by  a  unit  volume,  or  a  unit  cube  of  a  material 
taken  between  its  opposite  faces,  so  the  specific  magnetic 
reluctance  of  a  substance  is  best  defined  under  the  term 
reluctivity,  or  the  magnetic  reluctance  of  a  unit  cube,  i.  e., 
of  a  cubic  centimetre,  taken  between  parallel  faces.  The 
magnetic  reluctivity  of  vacuum  is  taken  as  unity,  and  the 
reluctivity  of  air,  copper,  wood  and  nearly  all  substances 
except  the  magnetic  metals,  does  not  differ  appreciably 
from  the  reluctivity  of  vacuum.  The  reluctivity  of  the 
magnetic  metals  varies  with  the  density  of  the  flux  tra- 
versing them. 

Fig.  41  shows  curves  of  reluctivity  in  various  samples 
of  iron  and  steel  for  different  flux  densities.  Thus  the 
lowest  curve  No.  VII. ,  representing  soft  annealed  Nor- 
way iron,  shows,  for  example,  a  reluctivity  of  0.7  thou- 
sandth, i.  e.y  -j-^-yth  that  of  air,  at  a  flux  density  of  10 
kilogausses.  In  other  words  each  cubic  centimetre  of  this 
iron  subjected  to  a  flux  intensity  of  (B  =  10,000  gausses, 


99 


ABSCISSAE:  FLUX  DENSITY,  GAUSSES (e©) 


TOTAL  MAGNETIC  INTENSITY  OR  FLUX  DENSITY  (§S)  GAUSSES 

FIG.  41. 

Curves  of  reluctivity  in  iron  and  steel  in  relation  to  flux  density,  from  measurements 
by  Kennelly. 


100 


offers,  between  opposed  faces,   a   reluctance  of  0.0007 
oersted. 

108.  There  are  three  varieties  of  magnetic  circuits ; 
viz., 

(1.)  The  non-ferric  circuit,  where  the  magnetic  circuit 
is  completed  through  air  or  other  non-magnetic  mate- 
rials. Such  would  be  the  magnetic  circuit  of  a  hollow 
coil  of  wire. 

(£.)  The  ferric  circuit,  where  the  magnetic  circuit  is 
entirely  completed  through  iron,  as  in  the  case  of  an 


FIG.  42. 

Non-ferric  magnetic  circuit.  Coils  of 
insulated  copper  wire  on  rubber  cylinders 
distributing  a  magnetic  circuit  through 
air,  wood  and  hard  rubber. 


FIG.  43. 

Ferric  magnetic  circuit,  an  alternating 
current  transformer.  With  the  exception 
of  "leakage"  all  the  flux  passes  through 
iron. 


iron  ring  wrapped  with  wire,  or  an  electro-magnet  with 
the  keeper  pressed  upon  its  poles. 

(3.)  The  aero-ferric  circuit,  in  which  the  circuit  lies 
partly  through  air  and  partly  through  iron.  To  this 
class  of  circuit  belong  the  great  majority  of  dynamos 
and  electro-magnetic  apparatus. 

Fig.  42  represents  a  type  of  non-ferric  circuit.     Fig. 


101 


43  represents  a  type  of  ferric  circuit ;  and,  Fig.  44,  a 
type  of  aero-ferric  circuit. 

109.  The  reluctances  of  practical  magnetic  circuits 
are  very  difficult  to  compute,  owing  to  the  varia- 
tion of  the  cross-section  of  the  magnetic  circuit  at  differ- 
ent points.  Fig.  45,  however,  shows  a  particular  case  of 
non-ferric  magnetic  circuit,  which  is  amenable  to  very 
simple  treatment. 

If  in  the  anchor  ring  of  wood,  copper,  or  other  non- 
magnetic material,  uniformly  wrapped  with  wire,  as 


FIG.  44. 

Aero-feme  circuit  ;  a  telegraph  relay.     Magnetic  circuit  through  cores,  yoke  and 
keeper  of  magnets  and  air  gaps  at  poles. 

shown  in  Fig.  45,  the  mean  circumference  of  the  coil  is 
50  cms.,  and  the  cross-section  of  the  interior  of  the  coil  is 
five  square  centimetres,  then  the  reluctance  of  the  mag- 
net circuit,  which  will  be  confined  entirely  to  the  space 

within  the  winding,  will   be   approximately  — -   =  10 

5 

oersteds.  All  the  flux  paths  in  this  case  will  be  circular, 
and  there  will  be  no  magnetic  flux  outside  the  winding. 
A  compass-needle,  therefore,  held  near  the  ring,  provided 
the  ring  be  uniformly  wrapped,  will  fail  to  show  whether 
the  current  is  flowing  through  the  winding  or  not.  This 


102 


is  the  only  known  ease  in  which   magnetic    flux  can   he 
readily  confined  to  ;i  determinate  path. 

110.  If  the  core  of  the  preceding  ring  be  replaced  by 
soft  iron,  then  the  reluctance  of  the  circuit  may 
be  1,000  times  less,  and,  consequently,  the  magnetic  flux 
in  the  circuit  a  thousand  times  greater.  Such  a  ring, 
although  carrying  a  powerful  magnetic  flux,  would  still 
evidence  no  external  magnetism,  hut  if  a  saw-cut  he 
made  through  the  ring  at  any  point,  as  in  Fig.  4(5,  the 
opposite  faces  of  this  gap  would  show  opposite  polarity, 
and  the  magnetic  circuit  would  then  become  of  the  aero- 


FIG.  45. 

Principal  sections  of  closed 
circular  coil  and  its  mag- 
netic circuit.  Core  of  wood 
or  iron. 


i.   46. 

Diagram  of  aero-ferric 
magnetic  circuit.  Anchor 
ring  iron  core  with  air-gap. 


Fio.  47. 

Diagram  of  aero-ferric 
magnetic  circuit.  Anchor 
ring  iron  core  with  wider 
air-gap. 


ferric  type,  with  flux  lines  preceding  through  the  sur- 
rounding air.  At  the  same  time,  the  magnetic  reluct- 
ance of  the  circuit  is  markedly  increased  ;  thus  if  the 
saw-cut  is  one  millimetre  in  width,  and  its  area  of  cross- 
section  five  square  centimetres,  the  increase  of  reluctance 

thus  added  to  the  previous  ferric  circuit  would  be  ~  = 

0.02  oersted  approximately.  The  true  value  of  the  re- 
luctance would  be  somewhat  less  than  this  owing  to  the 


103 


slight  diffusion  of  the  flux  beyond  the  limits  of  the  air 
gap  as  shown,  thereby  sensibly  increasing  the  effective 
cross-section  of  the  air  gap. 

111.  If  the  width  of  the  air  gap  be  increased,  as 
shown  in  Fig.  47,  then  the  increase  in  the  reluct- 
ance of  the  circuit  will  produce  a  still  more  marked 
variation  in  the  amount  of  magnetic  flux,  and  the  diffu- 
sion of  the  lines  will  be  more  marked.  Owing  to  this 
diffusion,  the  reluctance  of  the  air  gap  will  be  increased, 


FIG.  48. 

Diagram  of  aero-ferric  circuit  of  half  ring. 

but  not  in  proportion   to  its  length,  the  average  cross- 
section  being  greater  than  five  square  centimetres. 

If  the  air  gap  be  still  further  widened,  as  in  Fig.  48, 
the  same  effects  are  still  more  markedly  produced  until 
the  total  flux  in  the  magnetic  circuit  may  be  only  a 
small  fraction  of  that  existing  in  the  original  case.  But 
this  weaker  magnetic  flux  may  have  far  more  powerful 
influence  upon  neighboring  magnets,  owing  to  the  exter- 
nal diffusion  of  the  flux  paths,  as  shown. 


104 


It  is  a  curious  fact  that  although  the  reluctivity 
of  all  non-magnetic  substances  is  practically  the 
same  as  that  of  the  air-pump  vacuum,  yet  the  reluctivity  of 
different  specimens  of  iron  is  subject  to  marked  varia- 
tions. An  exceedingly  small  percentage  of  carbon  in 
iron  may  greatly  increase  its  reluctivity.  As  a  rule  the 
softer  and  purer  the  iron,  the  lower  its  reluctivity. 
Nickel  is,  perhaps,  the  only  ingredient  which  forms  an 
exception  to  this  rule. 

SYLLABUS. 

The  reluctivity  of  a  magnetic  circuit  is  the  resistance 
it  offers  to  the  passage  of  the  magnetic  flux  through  it 
under  a  given  M.  M.  F, 

Specific  reluctance,  or  reluctivity  of  a  substance,  is 
the  reluctance  offered  by  a  cubic  centimetre  of  the  sub- 
stance between  opposite  faces. 

Reluctance  is  measured  in  units  called  oersteds.  An 
oersted  is  the  reluctance  offered  by  a  cubic  centimetre  of 
air-pump  vacuum. 

Magnetic  circuits  are  of  three  kinds ;  non-ferric,  ferric, 
and  aero-ferric. 

The  reluctivity  of  iron  is  much  less  than  that  of  air, 
but  varies  with  the  flux  density;  at  first  diminishing 
and  afterwards  increasing  with  the  density. 

A  closed  circular  coil  is  the  only  form  of  magnetic 
circuit  in  which  the  flux  is  strictly  limited  to  a  definite 
path.  In  aero-ferric  circuits,  the  diffusion  of  the  mag- 
netic flux  will  be  greater  as  the  portion  of  the  circuit 
occupied  by  air  is  increased. 

Laboratory  of  Houston  &  Kennelly, 
Philadelphia. 


[Copyright,  1894,  by  THE  EI.FCTRICAL  ENGINEER.] 
WEEKLY. 

No.  14.  SEPTEMBER  15,  1894. 

Electrical   Engineering   Leaflets, 


Prof.  E.  J.  Houston,  Ph.  D. 

AND 

A.  E.  Kennelly,  F.  R.  A.  S. 


INTERMEDIATE    GRADE. 

MAQNETIC        FLUX. 


113.  The  magnetic  flux  in  any  magnetic  circuit  is 
directly  proportional  to  the  M.  M.  F.  acting  on  that 
circuit  and  inversely  proportional  to  its  reluctance  ;  or, 
since  the  unit  of  magnetic  flux  is  the  weber,  the  unit  of 
M.  M.  F.  the  gilbert,  and  the  unit  of  magnetic  reluctance 
the  oersted,  we  have  the  general  expression, 


webers  =  or,  9  =  *-, 

oersteds  (R 

this  corresponding  with  the  expression  given  by  Ohm's 
electric  circuit  law, 

volts          /       E 
amperes  =  —  -  or,  1  =  —  . 
ohms  R 

Here  SF,  is  the  existing  international  symbol  for  Mag- 
netomotive Force,  similarly,  (R,  denotes  Eeiuctance, 
and  the  0  is  the  symbol  for  Flux. 

In  either  of  the  above  equations,  any  two  of  the  three 

Published  by 

THE  ELECTRICAL  ENGINEER, 
203  Broadway,  New  Vork   N.  Y. 

[Entered  as  second-class  matter  at  the  New  York,  N.  Y.,  Post  Office,  June  14,  1894.] 


106 


independent  quantities  being  known,  the  remaining  one 
can  be  calculated.  Thus, 

gilberts  =  webers  X  oersteds  ;  SF  =  0  (R. 
Or, 

oeistedg=8aberte;«  =  g. 
webers  (P 

114.  In  practical  magnetic  circuits  it  is  often  a  matter 
of  considerable  importance  to  be  able  to  calculate 

the  magnetic  flux.  In  order  to  do  this,  in  accordance 
with  the  preceding  principles,  it  is  only  necessary  to  de- 
termine the  values  of  the  M.  M.  r.  and  the  reluctance ; 
for,  as  is  evident,  there  are  but  two  ways  in  which  the 
value  of  the  magnetic  flux  in  any  circuit  can  be  varied ; 
namely,  by  altering  the  value  of  either  of  these  quan- 
tities. 

The  value  of  the  M.  M.  F.  is  most  readily  increased  by 
increasing  the  strength  of  the  exciting  current.  We 
will  now  show,  by  some  practical  examples,  how  the 
preceding  equations  may  be  applied  in  the  determination 
of  magnetic  flux  in  a  circuit. 

115.  Case  1 — the   simple   case   of  an   anchor   ring, 
of    soft    Norway    iron,    wound   with    insulated 

wire  :  We  commence  with  this  case,  because,  as  we  have 
already  pointed  out,  this  type  of  circuit,  if  properly  con- 
structed, possesses  no  magnetic  leakage.  The  area  of 
cross-section  of  the  iron  ring,  of  dimensions  shown  in 
Fig.  49,  is  3.1416  square  inches  =  20.268  square  centi- 
metres, and  the  mean  length  of  the  circuit,  or  the  mean 
circumference  of  the  ring,  is  37.7  inches  —  95.74  centi- 
metres. If  the  total  flux,  which  it  is  desired  to  send 
through  this  circuit,  be  350  kilowebers,  it  is  required  to 


107 


determine  the  M.  M.  F.  which  must  be  applied  to  the  ring 
in  order  to  produce  this  flux. 

The  reluctance  of  the  circuit  is  obtained  as  follows : 
When  350,000  webers  are  transmitted  uniformly  through 
a  circuit,  the  cross-section  of  which  is  20.268  sq.  cms., 
the  flux  density  will  be  -3/o°  A0/  =  1?>270  gausses  =  17.27 
kilogausses ;  i.e.,  17.27  kilowebers  to  the  square  centi- 
metre. Referring  to  the  diagram,  Fig.  41,  it  will  be  seen 
on  Curve  No.  VII.,  that,  at  this  density,  the  reluctance  of 
a  cubic  centimetre  of  Norway  iron  is  by  measurement,  19.2 


FIG.  49. 

Ferric  Magnetic  Circuit.  Norway  iron  ring  uniformly  wound  with  wire  in  turns. 
Mean  circumference  37.7  ia.  =  95.74  cms.  Cross-section  of  ring  3.1416  sq.  in.  =  20.268 
sq.  cms. 

millioersteds.     The  reluctance  of  the  circuit  is,  therefore, 
X   19.2  =  90.7  millioersteds  =  0.0907  oersted. 


Inserting  this  reluctance  in  the  equation  £F  =  <#  (R,  we 
have  the  required  M.  M.  F.  =  350,000  X  0.0907  =  31,745 
gilberts.  Since  one  gilbert  is  0.8  ampere-turn  approxi- 
mately, the  required  number  of  ampere-turns  is  31,745 
X  0.8  =  25,396,  or  more  nearly  31,745  X  0.7958  = 
25,250  ampere-turns.  If  now  the  winding  of  the  ring 
be  composed  of  1,500  turns  of  insulated  wire,  the  cur- 


0?  THJS 


UFIVBRSIT7 


108 


rent  in  each  turn  must  be  *£,-£££-  =  1  T.I 6 7  amperes,  and 
tliis  is  the  required  exciting  current. 

116.  Case  2. — Taking  now  the  case  of  an  aero-ferric 
circuit,  in  which  part  of  the  flux-paths  lie  through 
air,  say,  an  electromagnet,  as  shown  in  Fig.  50,  let  us 
first  assume  that  the  leakage  is  sufficiently  small  to  be 
negligible ;  that  the  air-gap  in  the  circuit  is  fixed  ;  i.e.9 
that  the  keeper  cannot  move  up  to  the  poles  of  the  mag- 
net ;  and  that  the  iron  in  the  electromagnet  and  keeper 
is  ordinary,  soft,  wrought  iron.  Then  if  a  total  flux  of 


01-3  «J? 


£lec.Enffi*etr 


FIG.  50. 

Electromagnet  of  wrought  iron,  aero-ferric  circuit.     Air  gaps  %  in.  =  1.27  cm.     Mean 
length  of  magnetic  circuit  140.24  cms.     Cross-section  of  magnetic  circuit  25  sq.  cms. 

300  kilowebers  is  to  be  sent  through  the  circuit,  it  is 
required  to  find  the  excitation  necessary  for  the  magnet 
to  produce  this  flux.  Here,  as  before,  we  have  to  find 
the  total  reluctance  of  the  circuit.  Taking  first  the  air 
reluctance,  each  air-gap,  #,  «,  has  a  length  of  half  an 
inch  =  1.27  cms.;  and  a  cross-section  of  3.875  square 
inches  =  25  square  cms.  The  reluctance  of  each  air- 
gap  is,  therefore,  J^f-?  X  1  =  0.0508  oersted,  since  the 
reluctivity  of  air  is  unity.  Since  they  are  placed  in 
series,  the  reluctance  of  both  air  gaps  is,  therefore,  0.1016 
oersted. 


100 


If  the  cross-section  of  the  cores,  yoke  and  keeper  is 
uniformly  25  square  cms.,  the  flux  density  will  be  uni- 
formly $££-  =  12  kilogausses.  Referring  to  Fig.  41,  the 
reluctance  of  wrought  iron  at  this  density  may  be  taken 
as  approximately  1.316  millioersteds  in  a  cubic  centimetre. 
With  this  reluctivity  the  reluctance  of  the  iron  in  the 
circuit  will  be  i||^  X  1.316  =  7.248  millioersteds,  or 
0.007248  oersted,  since  the  mean  path  in  the  iron  has  a 
length  of  137.7  cms.  Adding  the  reluctance  of  the  air, 


FIG.  51. 

Electric  Circuit  Analogue  of  JEro- Ferric  Circuit  in  Fig.  50.    Without  Leakage. 

we  have,  for  the  total  reluctance  in  the  circuit,  0.108848 
oersted.  Consequently,  the  M.  M.  F.  required  will  be 
300,000  X  0.108848  =  32,654  gilberts  =  25,980  am- 
pere turns,  and,  should  the  number  of  turns  on  both 
coils  together  be  5,000,  the  required  exciting  current  is 
5.196  amperes. 

The  electric  circuit  corresponding  to  this  case  is  shown 
in  Fig.  51.     Here  two  equal  E.  M.  F.'S  in  series,  each  of 


110 


16,277  volts,  act  on  a  circuit  of  0.108848  ohm  resistance. 
The  drop  of  magnetic  potential  in  each  air-gap  is 
15,240  gilberts,  while  the  drop  of  potential  in  each  core 
is  453.7  gilberts. 

Owing  to  the  fact  that  the  air  round  the  magnet  is  not 
a  magnetic  insulator,  the  preceding  calculation  cannot 
be  regarded  as  strictly  correct,  since  we  have  left  all  ex- 
ternal, or  leakage  flux  out  of  consideration.  It  is  evident 
that  with  ferric  circuits,  in  which  the  flux  density  is  not 
excessive,  that  is  say,  in  which  the  reluctivity  of  the  cir- 
cuit is  far  less  than  that  of  the  external  air,  the  leakage 
will  be  small,  even  though  the  arrangement  of  the  cir- 
cuit differs  materially  from  the  anchor  ring  type  with 
uniform  winding.  As  the  air-gaps  in  a  circuit  become 
wider  and  more  numerous,  the  leakage  flux  bears  a 
larger  proportion  to  the  total,  and  the  circuit  becomes 
less  amenable  to  simple  numerical  treatment,  owing  to 
the  complexity  of  the  various  branch  circuits,  and  the 
difficulty  of  computing  their  local  reluctances.  This 
condition  renders  the  calculation  of  practical  magnetic 
circuits  much  more  tedious  and  difficult  than  that  of 
ordinary  electric  circuits.  Most  dynamo  or  motor  mag- 
netic circuits  can,  however,  be  computed  with  a  degree 
of  approximation  sufficient  for  practical  purposes  in  design. 
The  following  case  illustrates  the  method  of  procedure. 

117.  Case  3. — Let  the  magnet,  shown  in  Fig.  50. 
possess  a  leakage  flux  of  20  per  cent,  through 
the  path  5,  6,  7,  8.  That  is  to  say,  for  every  100  webers 
passing  through  the  cores  and  yoke,  20  pass  through  the 
air  between  the  poles,  and  only  80  pass  through  the 
keeper.  Eequired  the  M.  M.  F.  to  send  300  kilowebers 
through  the  keeper,  as  before. 


Ill 


The  flux  through  the  cores  and  yoke  will  now  be 
—  375  kilowebers,  at  a  mean  density  of  ^-f-  =15  kilo- 
gausses.  By  reference  to  Fig.  41,  the  reluctance  of  a  cubic 
centimetre  of  wrought  iron  for  this  density  is  3.077  milli- 
oersteds.  Of  the  25  square  cms.  of  cross-section  in  the 
cores  and  yoke,  only  20  can  now  be  considered  as  carrying 
the  keeper  flux,  the  remainingfive  square  cms.  being  allot- 
ted to  the  leakage  flux.  Since  the  mean  length  of  circuit 


15.240  Volts 
0.0608  Ohm 
300,000  Amp.-, 


FIG.  52. 

Electric  Circuit  Analogue  of  ^Ero-Ferric  Circuit  in  Fig.  50.    With  Leakage. 

through  cores  and  yoke  is  98.85  cms.,  their  reluctance  in 
the  main  or  keeper  circuit  is  &-^$-8-  X  3.077  =  15.21  milli- 

oersteds  =    >.  . . .   0.01521  oersted. 

The  reluctivity  of  the  keeper  remains 
at  1.316  millioersteds,  as  its  flux  den- 
sity is  12  kilogausses.  The  keeper 
reluctance  is,  therefore,  3||3  X  1.316 

=  2.045  millioersteds  =     0.002045      " 

The  two  air-gap  reluctances  remain  as 

before  at .  .  . .  0.1016          " 


So  that  total  reluctance  in  keeper,  etc.  =  0.118855  oersted. 


112 


The  M.  M.  F.  necessary  to  force  300  kilowebers  through 
this  circuit  is  300,000  X  0.118855  =  35,656.5  gilberts 
=  28,370  ampere-turns,  or  14,185  ampere-turns  to  each 
spool,  requiring  a  current  strength  of  5.674  amperes. 

The  corresponding  electric  circuit  is  shown  in  Fig.  52. 
It  will  be  observed  that  while  the  drop  of  pressure  in 
the  air-gaps  is  15,240  gilberts,  as  before,  the  drop  in  the 
cores  and  yoke  has  been  increased  by  the  introduction  of 
leakage  from  1560.9  gilberts  to  4562  gilberts. 

The  reluctance  of  the  leakage  path  between  the  poles 
is  observed  to  be  0.4146  oersted.  It  is  evident,  there- 
fore, that,  whenever  the  reluctance  of  leakage  paths  can 
be  computed,  the  distribution  and  amount  of  leakage 
flux  can  be  determined. 

SYLLABUS. 

In  any  magnetic  circuit  the  webers  =  the  gilberts 
divided  by  the  oersteds.  Corresponding  to  Ohm's  law 
in  the  electric  circuit,  the  amperes  —  the  volts  divided 
by  the  ohms 

Given,  any  two  of  the  three  quantities  in  either  oi' 
the  above  formulae,  the  value  of  the  other  quantities- 
may  be  calculated. 

The  value  of  the  magnetic  flux  in  any  circuit  may  be 
increased  either  by  increasing  the  M.  M.  F.  or  by  decreas- 
ing the  reluctance.  The  M.  M.  F.  may  be  increased  by 
increasing  the  number  of  ampere-turns  in  the  magnetiz- 
ing circuit. 

In  the  design  of  electromagnets,  the  reluctance  can 
be  varied  either  by  varying  the  dimensions  of  the  iron  cir- 
cuit, or  by  varying  the  character  of  the  iron  employed. 

Laboratory  of  Houston  &  Kennelly, 
Philadelphia. 


[Copyright,  1894,  by  THE  ELECTRICAL  ENGINEER.] 
WEEKLY. 

No.  15.  SEPTEMBER  22,  1894. 

Electrical   Engineering   Leaflets, 


— BY— 

Prof.  E.  J.  Houston,  Ph.  D. 

AND 

A.  E.  Kennelly,  F.  R.  A.  S. 


INTERMEDIATE    GRADE. 

ELKCTROMAQNBTS, 


118.  An  electromagnet  is  a  magnet  produced  by  the 
passage  of  an  electric  current  through  a  coil  of 

r  o  D 

wire  linked  with  a  magnetic  circuit.  The  name  electro- 
magnet is  practically  limited,  however,  to  cases  where 
the  core,  placed  inside  the  helix,  is  made  of  soft  iron. 
Under  these  circumstances  the  core  acquires  the  proper- 
ties of  an  electromagnet,  and,  disregarding  residual  mag- 
netism, loses  these  properties  when  the  current  ceases. 

119.  When  a  bar  of  hard  or  soft  iron  is  brought  into 
a  magnetic  flux,  an  alignment  of  its  molecules,  or 

ultimate  particles,  is  supposed  to  take  place.  This  align- 
ment is  more  readily  produced  in  soft  iron  than  in  hard- 
ened iron,  as,  indeed,  would  be  supposed,  bearing  in  mind, 
the  characteristic  property  of  hard  iron  which  opposes 
any  deformation  or  change  of  shape.  When  the  prime 
M.  M.  r.  ceases  to  act  on  the  iron,  as  would  occur  either 
by  withdrawing  the  iron  core  from  the  prime  flux,  or  by 
causing  the  magnetizing  current  to  cease,  the  freedom  of 

Published  by 

THE   ELECTRICAL  ENGI 
203  Broadway,  New  Vork  N 

[Entered  as  second-class  matter  at  the  New  York,  N.  Y., 


114 


movement  naturally  possessed  by  the  molecules  of  soft 
iron,  permit  them  readily  to  lose  their  new  alignment, 
and  the  structural  M.  M.  F.  is  dissipated  with  the  resump- 
tion of  practically  its  previous  condition.  In  the  case  of 
hardened  steel,  however,  the  resistance  which  the  mole- 
cules offer  to  change  of  position,  enables  the  structural 
M.  M.  F.  to  be  largely  retained.  For  this  reason  the  mag- 
netism produced  in  soft  iron  is  sometimes  called  tempor- 
ary magnetism,  and  that  in  hard  steel,  permanent 
magnetism. 


IV 


V  VI 

FIG.  53. 

Indicating  the  direction  of  magnetization  in  an  iron  bar  as  dependent  on  the  direction 
of  winding  and  of  current. 

120.  When  an  electric  current  passes  through  a  wire, 
an  M.  M.  F.  is  established  around  the  wire.  This 
M.  M.  F.  produces  a  distribution  of  flux  in  cylinders  con- 
centric to  the  wire,  the  intensity  diminishing  directly 
with  the  distance  from  the  axis  of  the  wire.  The  di- 
rection of  this  flux,  relative  to  the  direction  of  the  current 
in  the  wire,  being  as  shown  in  Fig.  53.  When  the  wire 
is  bent  into  a  circular  loop,  it  is  evident  that  the  M.  M.  F. 
produced  by  the  loop  is  directed  either  all  upwards  or 
all  downwards  through  the  loop,  so  that  the  direction  of 
the  flux  depends  on  the  direction  of  the  current.  The 


115 


M.  M.  F.  from  a  helix,  which  lias  a  succession  of  turns,  is 
also  directed  through  the  helix  in  one  direction  or  the 
other,  both  according  to  the  direction  of  the  current 
and  to  the  direction  of  the  winding. 

121.  Magnets  may  be  divided  into  different  classes 
according  to  the  character  of  the  work  they  are 

called  upon  to  perform  ;  namely, 

(1.)  Tractive  magnets ;  and, 

(2.)  Portative  magnets. 

A  tractive  magnet  is  one  designed  to  exert  a  pull  at  a 
distance.  A  portative  magnet  is  one  designed  to  hold 
or  support  heavy  weights  attached  to  its  armature,  when 
the  latter  is  at  rest  upon  the  poles. 

122.  An  electromagnet  is  designed  to  produce  a  cer- 
tain  traction  or   pull   on  its  keeper.      This  pull 

may  be  exerted  either  when  the  keeper  is  at  a  distance ; 
that  is,  separated  by  an  air-gap ;  or,  when  the  keeper  is 
actually  brought  into  contact  with  the  polar  surfaces.  In 
most  practical  cases,  however,  the  attractive  force  is 
brought  to  bear  between  two  parallel  surfaces,  usually 
called  the  polcvr  surfaces  across  which  the  flux  passes 
perpendicularly,  as  shown  in  Fig.  54.  Under  these  cir- 
cumstances, every  square  centimetre  of  opposed  polar 

(B2 
surfaces  attracts  the   other   with  a  force  of  —  dynes, 

where  (E  is  expressed  in  gausses,  so  that  if  the  intensity 
is  everywhere  the  same  across  the  surfaces  A,  B,  c  and  D, 
E,  F,  the  total  force  of  attraction  between  the  surfaces 

(&2 
will  be  8  X  -  -  dynes.     S  being  the  area  of  polar  sur- 

8  7T 

face  in  square  cms. 


116 


123.  If  the  flax  does  not  possess  the  same  value  at 
different  portions  of  the  polar  surfaces,  then  the 
active  surface  must  be  divided  into  a  sufficient  number 
of  elements  to  permit  the  flux  density  to  be  considered 
uniform  over  each  element,  when  the  separate  forces  of 
each  element  can  be  determined,  and  their  sum  will  be 
the  total  force  on  the  whole  surface.  So  far  as  the  at- 
tractive power  of  a  magnet  is  concerned,  the  total  value 
of  the  flux  is  of  secondary  importance ;  it  is  the  distri- 


FIG.  54. 

Flux  normal  to  opposed  plane  parallel  polar  surfaces. 

bution  of  the  intensity  of  the  flux  at  the  active  surfaces, 
in  gausses,  which  is  of  primary  importance.  In  soft 
Norway  iron,  the  flux  intensity  can  hardly  be  maintained 
above  19  kilogausses.  The  attraction  between  opposed 
surfaces  of  one  square  cm.  in  area  traversed  perpendi- 
cularly by  19  kilowebers,  will  be,  therefore, 
19,000  X  19,000  =  d 

25.133 
and,  since  one  dyne  equals  1.0203  milligrammes  weight, 


11 


at  Washington  this  force  represents  14,658  grammes,  or 
32.31  pounds  weight,  at  "Washington,  per  square  centi- 
metre of  active  polar  surface  (208.4  pounds  per  square 
inch). 

124.  In  Fig.  40  on  page  94,  a  sextipolar  dynamo  is 
represented  in  cross-section.  Since  flux  passes 
into,  or  out  of,  the  armature  beneath  each  polar  surface, 
each  magnet  core  may  be  said  to  attract  the  armature, 
with  a  force  that  can  be  readily  computed,  to  at  least  a 
fair  degree  of  approximation,  when  the  elements  of  the 
magnetic  circuit  are  known. 

For  example,  suppose  that  in  each  of  the  six  magnetic 
circuits  shown,  the  useful  flux,  i.e.,  the  flux  passing 
through  the  armature  core,  is  two  megawebers.  Then 
four  megawebers  will  enter  or  leave  the  armature  under 
each  pole-piece.  If  the  surface  of  each  pole-piece  is  200 
square  inches,  i.e.,  1,293  square  cms.,  the  mean  intensity 
in  the  air-gap  and  polar  surfaces  will  be 

4>QQO>QQO  =  3,094  gausses. 
1,293 

Assuming  that  this  intensity  is  Uniform  over  the  sur- 
faces, the  tractive  force,  per  square  centimetre,  exerted 
between  pole  and  armature  will  be 

3,094 X3,094_38(^90()  dvnes=388>5  grammes  weight, 

.25.  loo 

and,  since  there  are  1,293  square  cms.  opposed  and  active, 
the  total  force  will  be 

388.5  X  1,293  =  502,300  gms.  weight  =  1,108  Ibs.  weight. 
As  the  armature  revolves,  therefore,  its  iron  core  will  be 
pulled  upon  outwards  opposite  each  pole,  with  a  force 
of  about  half  a  ton's  weight,  and  the  framework  sup- 
porting the  armature  must  be  sufficiently  strong  to  safely 


118 


support  these  forces,  in  addition  to  the  ordinary  centri- 
fugal forces  of  rotation. 

125.  When  a  current  passes  through  the  armature, 
whether  it  be  acting  as  a  generator  or  motor,  the 
effect  of  the  M.  M.  F.,  set  up  by  the  current  in  the  arma- 
ture windings,  is  to  superpose  its  flux  upon  that  previously 
established  by  the  field  magnet  M.  M.  F.'S.  The  combina- 
tion of  the  two  magnetic  circuits  is  to  destroy  the  sym- 
metry of  the  flux  distribution.  For  example,  if  the 
machine  is  receiving  current  as  a  motor,  the  effect  of 
introducing  the  armature  M.  M.  F.'S  will  be  to  so  modify 
the  flux  distribution,  shown  in  Fig.  40,  as  to  increase  the 
intensity  in  the  air-gaps  underneath  all  the  left-hand 
edges  of  the  polepieces  (as  each  pole  is  regarded  from 
the  armature),  and  reduce  the  intensity  at  the  opposite 
or  right-hand  edges.  At  the  same  time,  the  flux  will  be 
deflected  from  the  perpendicular,  and  drawn  through 
the  air  more  or  less  obliquely.  Under  these  circum- 
stances tangential  pulls  will  be  exerted  upon  the  arma- 
ture, and  each  square  centimetre  on  the  left-hand  edge 
will  exert  an  increased  pull  in  proportion  to  the  square 
of  the  intensity,  while  the  right-hand  polar  edge  will 
exert  a  pull  which  is  relaxed  in  corresponding  measure. 
The  resultant,  or  preponderating  forces  on  the  left-hand 
polar  edges,  will  draw  the  armature  round  clockwise. 
This  effect  of  the  M.  M.  F.  in  the  armature  is  called 
armature  reaction.  It  is  by  reason  of  armature  reaction 
that  a  motor  pulls,  and  that  a  generator  has  to  be  pulled, 
while  the  pull  is  in  all  cases  a  distribution  of 

—  dynes  per  square  cm. 
over  the  opposed  polar  surfaces,  under  distortion  from 


10 


the  original  symmetry  of  flux  distribution.  The  funda- 
mental law  of  tractive  force  in  the  electromagnet  is  con- 
sequently the  fundamental  law  of  rotary  force  in  all 
electric  dynamos  and  motors. 

126.  The  portative  force,  which  a  magnet  can  exert, 
may  readily  reach  200  Ibs.  weight  per  square  inch 

of  active  polar  surface  when  the  poles  are  of  soft  iron. 
It  should,  therefore,  be  the  object,  when  designing  a  ferric 
magnetic  circuit  for  simply  portative  purposes,  to  magneti- 
cally saturate  the  polar  surfaces  as  nearly  as  possible,  and 
not  allow  the  iron  to  become  equally  saturated  at  any  other 
part  of  the  circuit.  For  this  reason  it  is  usual  to  con- 
strict the  section  of  the  iron  at  the  poles.  The  length  of 
the  circuit  is  then  reduced  as  far  as  possible,  so  as  to 
only  allow  just  room  for  the  exciting  coil.  In  this  way 
a  very  small  electro-magnet  weighing  a  decigramme  can 
be  made  to  support  2,500  times  its  own  weight,  a  magnet 
weighing  100  grammes,  600  times  its  own  weight,  and  a 
multipolar  magnet,  weighing  a  ton,  should  be  able  to 
support  about  500  times  its  own  weight. 

When  a  tractive  magnet  has  to  exert  a  definite  pull, 
under  a  given  M.M.  F.  upon  its  armature,  across  an  air-gap, 
the  design- of  the  magnetic  circuit  has  to  be  altered.  It 
is  found  that  the  best  area  of  polar  surface  to  employ  is 
that  which  makes  the  reluctance  of  the  air  equal  to  the 
reluctance  of  the  iron.  This  rule  ensures  the  best  in- 
tensity in  the  polar  surfaces  assuming  no  leakage  to  exist. 
The  influence  of  leakage  calls  for  a  reduction  in  the  air 
reluctance. 

127.  When   a  tractive   magnet   has    to    alternately 
attract  and  release  its  armature  in  rapid  succes- 
sion, as,  for  example,  in  the  case  of  a  telegraph  relay,  the 


120 


armature  lias  to  be  made  very  light,  in  order  that  its  in- 
ertia may  not  unduly  increase  the  magnetic  forces  to  be 
exerted. 

An  ordinary  Western  Union,  neutral  relay  of  140 
ohms  resistance,  has  about  7,500  turns  of  wire,  and,  when 
excited  by  a  steady  current  of  25  milliamperes,  i.e.,  to  a 
M.  M.  F.  of  1ST. 5  ampere-turns,  or  235.8  gilberts,  exerts 
a  total  pull  upon  the  face  of  its  armature  amounting  to 
about  78  grammes  weight  when  the  distance  between 
poles  and  armature  is  0.1  cm.  The  reluctance  of  its 
circuit  is  then  about  0.25  oersted,  and  the  flux,  con- 
sequently, about  943  webers. 

SYLLABUS. 

When  a  bar  of  soft  iron  is  submitted  to  the  permeat- 
ing action  of  a  magnetic  flux,  its  ultimate  particles  be- 
come aligned,  and  a  structural  M.  M.  F.  is  established  in 
the  bar.  On  the  withdrawal  of  the  permeating  mag- 
netic flux,  the  structural  M.  M.  F.  disappears,  through  in- 
instability.  Hard  iron  or  steel  retains  the  structural 
M.  M.  F.  in  a  greater  or  less  degree. 

The  tractive  force   exerted  between   opposed   plane 

/n2 

parallel  polar  faces  is  -  -  dynes  per  square  cm.  of  either, 

8  7T 

or  0.2567  &2  dynes  per  square  inch,  or  5.771&X  10~7  Ibs. 
per  square  inch,  (B  being  expressed  in  gausses. 

The  electromagnetic  rotary  pull  of  a  dynamo  or  motor 
is  due  to  tractive  forces  set  up  between  the  armature  and 

(&2 
field  poles  represented  locally  by  -  -   dynes  per  square 

8  7T 

cm.  from  point  to  point. 

Laboratory  of  Houston  &  Kennelly, 
Philadelphia. 


[Copyright,  1894,  by  THE  ELECTRICAL  ENGINEER.] 
WEEKLY. 

No.  16.  SEPTEMBER  29,  1894. 

Electrical   Engineering   Leaflets, 


Prof.  E.  J.  Houston,  Ph.  D. 

AND 

A.  E.  Kennelly,  F.  R.  A.  S. 


INTERMEDIATE   GRADE 

INDUCED    EX  JVL 


128.  Whenever  a  conductor  moves  across  a  magnetic 
flux,  or  a  magnetic  flux  moves  across  a  conductor, 

an  E.  M.  F.  is  generated  in  the  conductor ;  or,  generally, 
whenever  relative  motion  exists  between  a  conductor 
and  magnetic  flux  whereby  either  crosses  the  other,  an 
E.  M.  F.  is  generated  in  the  conductor.  The  amount  of 
this  E.  M.  F.  is  dependent  on  the  rate  of-  cutting  of  flux, 
and  will  evidently  vary  both  with  the  rapidity  of  mo- 
tion of  either  the  flux  or  the  conductor  and  with  the 
intensity  of  'the  flux. 

129.  The  following  varieties  of  induced  E.  M.  F.  come 
under  the  above  general  head ;  namely, 

(a.)  Dynamo-electric  induction,  where  a  conductor  is 
moved  across  magnetic  flux. 

(b.)  Magneto-electric  induction,  where  magnetic  flux 
is  moved  across  a  conductor  by  the  motion  of  a  magnet. 

(<?.)  /Self-induction,  where  magnetic  flux  generated  by 
a  circuit  moves  through  the  circuit. 

Published  by 

THE  ELECTRICAL  ENGINEER, 
203  Broadway,  New  York  IN.  Y. 

[Entered  as  second-class  matter  at  the  New  York,  N.  Y.,  Post  Office,  June  14,  1894.] 


122 


(d.)  Mnt.mil  ////A/'-//////,  win-re  njMjrnetjr  tlnx  gene 
hy  on<-  fin-nit  move.H  through  ?i  nei-jlihonii;/  circuit. 

130.     Three   c;iHei   may    arise    when-  i..    M.    jr.    i-    pro 
duced    by    tin-    motion    of  ;i    conductor    through 
magnetic  flux.      Fir.-t,  where  a  conductor  at  ri^-ht  jingle- 
to  the  flux,   moves   in  a  <lin-ction  at    j-i^lit   ;ui<j:. 

••oiul,  \vJicn-a  conductor  olili'jm-.  to  tin-  jlux, 
moved  in  a  direction  at  right  ;ingl<-.-  to  tin-  flux.  rJ'liir<l, 
where  a  conductor,  oblique  to  the  flux,  mov<-  in  u 

ohlirjn<;    to    tlici  flux.       The    hist,   is    tin;    rnoHt 


Fn;.  5.-,. 


FIG.  56. 


.  57. 


Conductor  normal  to  flux.  Conductor  oblique  to  flux.  Conductor  ol.liq.  v  -\»  flux. 
moving  in  direction  normal  moving  in  direction  normal  moving  >n  direction  oblique 
to  flux.  to  flux.  to  flux. 

general  case.  Fig.  55  shows  the  first  case,  where  a 
straight  wire  A  B,  of  length  I  cms.  moves  through  a  uni- 
form flux  with  a  velocity  of  v  cms.  per  second,  which 

would  (-firry  it  in  oru-  second  to  the  position  A'  u'.  Tin-. 
flux  cutthrough  in  this  time  would  be  that  panning  through 
the  rectangle  A  B  B'  A'.  Here  the  total  flux  cut  will  be 
the  area  of  this  rectangle  in  sq.  cmn.  multiplied  l>\  the 
intensity  in  gausses,  and  will  vary  with  three  <|n;intiti«-.- : 
viz.,  the  length  of  the  wire,  the  velocity  of  the  mot  inn. 
and  the  intensity  of  the  flux. 


123 


131.  In  the  second  Ctte,  as  shown  in  Fig.  50,  where 
a  conductor,  oblique  to  the  flux,  is  moved  in  a 
direction  at  right  angles  to  it,  the  j;.  M.  i -.  generated  will 
depend  on  the  amount  of  flux  cut  per  second,  and  this 
will  be  equal  to  the  flux  passing  through  the  rectangle 
A  It  t/  A',  where  the  side  A  £>,  is  the  virtual  length  of  A  B, 
that  is,  its  length  projected  at  right  angles  to  the  flux, 
and  the  projected  length  will  evidently  be  smaller,  the 
greater  the  angle  /9,  or  the  greater  the  obliquity  to  the  flux. 

In  the  third  case,  shown  in  Fig.  57,  both  conductor 
and  motion  are  oblique  to  the  flux,  and  the  E.  M.  F.  in 
the  conductor  is  proportional  to  the  amount  of  flux  con- 
tained in  the  rectangle  A  b  V  of,  where  A  J,  is  the  virtual 
or  projected  length  of  the  conductor,  and  A  a',  its  virtual 
velocity. 

The  direction  of  the  E.  M.  F.  produced  by  the  movement 
of  a  conductor  across  magnetic  flux  is,  perhaps,  most 
readily  determined  by  Fleming's  hand  rule,  in  which, 
if  the  right  hand  be  held  as  shown  in  Fig.  58,  then  if  a 
conductor  be  moved  in  the  direction  in  which  the  thumb 
points,  and  at  right  angtes  to  the  flux  in  the  direction 
pointed  out  by  the  fore-finger,  the  E.  M.  F.  generated  will 
flow  in  the  direction  pointed  out  by  the  middle  finger. 

For  example,  if  a  straight  wire  10  cms.  long,  make  an 
angle  of  30°  with  the  flux,  whose  intensity  is  500 
gausses,  and  if  it  moves  at  a  rate  of  30  cms.  per  second, 
in  a  direction  making  an  angle  of  30°  with  the  flux 
paths,  the  virtual  length  of  the  wire  considered  as  lying 
across  the  flux  would  be  0.5  X  10  =  5  cms.,  and  the  virtual 
velocity  of  cutting  the  flux  would  be  0.5  X  30  =  15  cms. 
per  second,  so  that  the  E.  M.  F.  induced  in  the  wire  would 
be  5  X  15  X  500  =  37,500  c.  o.  s.  units,  =  375  micro- 


124 


volts.  This  E.  M.  F.  would  be  produced  during  the  motion 
of  the  wire,  but  would  cease  the  moment  the  wire  came 
to  rest. 

132.     In  order  that  an  induced  E.  M.  F.  may  set  up  a 
current  in  a  conductor,  the  circuit  of    that  con- 
ductor must  be  closed ;  i.  e.,  it  must   form  a  conduct- 
ing loop.    If  portions  of  this  loop  are  cutting  across  mag- 


FIG.  58. 

netic  flux,  and  thereby  generating  E.  M.  F.  around  the  wire, 
the  loop  must  either  be  enclosing  more,  or  less,  flux.  If, 
therefore,  in  a  conducting  loop  all  the  elementary  por- 
tions be  cutting  through  flux  at  an  aggregate  rate  of  100 
millions  of  webers  per  second,  the  flux  added  to,  or  sub- 
tracted from,  the  loop,  will  be  100  million  webers  per 
second,  and  the  E.  M.  F.  around  the  loop  will  be  one  volt. 


125 


In  practice,  when  a  dynamo  armature  is  revolving  in  the 
magnetic  flux  established  by  its  field  magnets,  the  loops 
of  conductors  on  the  armature  are  having  flux  poured 
into  them  and  then  poured  out  of  them  successively ; 
that  is,  are  having  E.  M.  F.'S  induced  in  them  in  one 
direction  as  the  flux  is  pouring  in,  and  in  the  opposite 
direction  as  the  flux  is  pouring  out.  For  example,  if 
the  flux  through  a  dynamo  armature  be  100  megawebers, 
and  the  revolving  armature  be  so  constructed  that  it 
poured  all  this  flux  through  a  loop  upon  the  armature 
surface  at  a  uniform  rate  during  one-tenth  of  a  second, 
the  rate  of  pouring  flux  into  the  loop  during  that  period, 
would  be  1,000  megawebers  per  second,  and  the  E.  M.  F. 
existing  in  the  loop  during  the  same  period  would  be  10 
volts.  If  then,  during  revolution,  the  armature  con- 
tinued emptying  the  loop  at  the  same  rate  in  the- next 
tenth  of  a  second,  the  E.  M.  F.  in  the  loop  during  that 
period  would  be  still  10  volts,  but  in  the  reverse  direction. 

133.  All  forms  of  dynamo  electric  machinery,  that  is, 
all  forms  of  machinery  for  the  generation  or  modi- 
fication of  E.  M.  F.,  are  devices  whereby  magnetic  flux  is 
poured  into  and  emptied  out  of  conducting  loops.     The 
underlying  principles  are  always  the  same,  although  the 
methods  adopted  are  very  varied  in  detail. 

134.  It  is  important  to  point  out  that  the  magnitude 
of  the  E.  M.  F.  induced  in  a  conducting  loop  does 

not  depend  upon  the  total  flux  which  may  be  poured 
into  the  loop,  but  upon  the  rate  at  which  the  entry  and 
exit  are  made.  A  large  total  flux,  entering  slowly  into  a 
loop,  may  produce  less  E.  M.  F.  in  magnitude  than  a  small 
total  flux  entering  rapidly.  For  this,  reason,  the  E.  M.  F. 


126 


generated  by  a  dynamo  increases  with  an  increase  in  the 
speed  of  revolution  of  its  armature. 

135.  A  resultant  E.  M.  F.  is  never  produced  in  a  con- 
ducting loop,  by  its  passage  through  flux,  unless 
the  amount  of  flux  entering  the  loop  is  different  from 
that  leaving  it.  Thus,  if  the  conducting  ring  A  B  c,  in 
Fig.  59,  with  its  plane  at  right  angles  to  the  uniform 
flux,  be  moved  at  right  angles  to  the  flux,  then  it  will 
have  no  resultant  E.  M.  F.  generated,  since  the  flux  it  en- 


Elec.Engineer 


FIG.  59.  FIG.  60. 

Conducting  ring  normal  to  flux,  mov-  Rotation  of  a  loop  in  a  magnetic  flux, 

ing  in  plane  normal  to  uniform  flux. 

closes  at  any  instant  is  always  the  same.  Or>  since  the 
amount  of  flux  cut  by  the  advancing  edge  and  there- 
fore entering  the  loop,  is  equal  to  the  amount  of  flux 
cut  by  the  following  edge  and  therefore  leaving  the  loop, 
the  equal  and  opposite  E.  M.  F.  generated  in  these  two 
halves  of  the  ring  exactly  neutralize. 

136.     If,  however,  a  conducting  loop  be  rotated  in  a 
magnetic  field,  a  resultant  E.  M.  F.  will  be  gene- 
rated in  it.     If,  for  example,  the  loop  A  B  c,  be  rotated 


127 


round  the  axis  H  K,  Fig.  60,  it  will,  in  different  positions, 
have  an  amount  of  flux  poured  into  it  differing  from 
that  poured  out  from  it.  If  at  any  instant,  the  rate 
at  which  flux  is  being  emptied  or  introduced  were  con- 
tinued unchanged  for  one  second,  the  amount  of  flux 
(webers)  leaving  or  entering  in  that  time,  would  be  the 
resultant  E.  M.  F.,  in  c.  G.  s.  units,  round  the  loop  at  that 
instant.  Fig.  61  shows  a  device  whereby  an  E.  M.  F.  can 


FIG.  61. 

E.  M.  F.  Produced  by  Rotation  of  Coil  in  Earth's  Flux. 

be  obtained  from  the  earth's  magnetic  flux,  by  revolving 
a  coil  of  many  turns,  in  a  supporting  frame. 

137.  When  the  circuit  of  a  voltaic  cell  is  closed 
through  a  long  coil  of  insulated  wire,  an  electro- 
motive force  is  developed  in  the  wire  by  the  flux  linked 
with  the  magnetic  circuit  established  under  the  M.  M.  F. 
of  the  active  conductor,  and  this  induced  E.  M.  F.  has  a 
direction  opposed  to  the  E.  M.  F.  of  the  battery.  When, 
however,  the  circuit  of  the  battery  is  broken,  owing  to 


128 


the  disappearance  of  the  flux  from  the  loops  of  the  cir- 
cuit, which  has  the  same  effect  as  the  linking  of  flux 
with  the  loop  in  the  opposite  direction,  an  induced  E.  M.  F. 
is  produced  in  the  wire,  whose  direction  is  the  same  as 
that  of  the  E.  M.  F.  of  the  battery.  These  phenomena 
are  called  the  phenomena  of  self-induction.  The  ten- 
dency of  the  E.  M.  F.  so  produced  is  to  oppose  the 
change  of  current  which  sets  it  up. 

138.  When  two  conducting  circuits  are  placed  in 
each  other's  vicinity,  and  an  electric  current  is 
established  in  one  of  them,  during  the  time  the  current  is 
increasing  in  strength,  the  flux  it  produces  links  with  the 
neighboring  circuit  and  develops  in  it  an  E.  M.  F.  which 
is  called  an  E.  M.  F.  of  mutual  induction. 

SYLLABUS. 

"When  a  relative  motion  takes  place  between  a  con- 
ducting circuit  and  a  magnetic  flux,  an  E.  M.  F.  is  pro- 
duced in  the  circuit,  varying  in  direction  with  the  direction 
of  the  motion.  The  E.  M.  F.  so  developed  is  called  in- 
duced E.  M.  F.  and  is  equal,  in  c.  G.  s.  units,  to  the  total 
amount  of  flux  that  is,  or  would  be,  cut  in  one  second  of 
time.  The  E.  M.  F.  of  induction  may,  therefore,  be  in- 
creased by  increasing  the  velocity  of  the  movement  or 
the  intensity  of  the  flux. 

If  the  total  flux  linked  with  a  circuit,  including  all 
the  loops  it  may  have,  be  expressed  in  webers,  the  E.  M.  F. 
in  volts,  induced  in  that  circuit,  at  any  moment,  will  be 
the  rate  at  which  the  flux  is  changing,  divided  by 
100,000,000. 

Laboratory  of  Houston  &  Kennelly, 
Philadelphia. 


[Copyright,  1894,  by  THE  ELECTRICAL  ENGINRKR,") 
WEEKLY. 

No.  17.  OCTOBER  6,  1894. 

Electrical   Engineering   Leaflets, 


—  BY— 

Prof.  E.  J.  Houston,  Ph.  D. 

AND 

A.  E.  Kennelly,  F.  R.  A.  S. 


INTERMEDIATE  GRADE 

DYNAMO. 


139.  A  dynamo-electric  machine  is  a  device  for  pro- 
ducing E.  M.  F.  by  successively  filling  and  empty- 
ing loops  of  wire  with  magnetic  flux.  A  dynamo-electric 
machine  consists  essentially  of  two  parts ;  namely, 
(1)  That  called  the  field,  which  produces  the  magnetic 
flux;  and  (2)  That  called  the  armature, which  bears  the 
loops  which  are  successively  filled  and  emptied  with 
flux. 

Numerous  machine  designs  have  been  produced  by 
which  these  objects  can  be  accomplished,  thus  giving 
rise  to  numerous  classes  of  dynamo-electric  machines. 

The  function  of  the  field  magnets  is  to  provide  the 
magnetic  flux  in  its  magnetic  circuit,  and  the  loops  of 
wire  on  the  armature  are  either  carried  through  this  flux, 
or  the  flux  carried  through  them,  so  that  they  are  suc- 
cessively filled  and  emptied.  The  rate  at  which  each 
loop  is  filled  and  emptied  determines  the  jalue  of  the 
E.  M.  F.  generated  in.  it,  and  the  number  of  such  loops 

Published  by 

THE   ELECTRICAL  ENGINEER, 
203  Broadway,  New  Vork   N.  Y. 

[Entered  as  second-class  matter  at  the  New  York,  N.  Y.,  Post  ok*,  J^ei* jf8g£|  ,'  f 

1  A. 


130 


with  their  grouping  or  connection,  the  total  amount  of 
E.  M.  F.  generated  by  the  machine. 

140.  In  the  United  States,  continuous  current  dyna- 
mos have  their  armatures  either  of  the  drum- 
wound  or  ring- wound  type.  An  example  of  the  drum- 
wound  type  of  armature  is  represented  in  cross-section 
at  Fig.  39,  page  93,  and  in  perspective  in  Fig.  62. 

Let  us  consider  two  loops,  c  D  E  F,  G  H  j  K,  Fig.  03  (a\ 
in  a  drum-wound  armature,  at  right  angles  to  eack  other, 
and  without  being  connected  to  a  commutator.  These 


FIG.  62. 

Drum  Armature. 

loops  are  supposed  to  be  supported  on  the  axis  A  B,  in 
a  bipolar  flux  represented  as  uniform  by  the  arrows. 

It  will  be  seen  from  an  inspection  of  the  figure,  that 
the  two  loops  being  at  right  angles,  one  is  filled 
with  flux,  and  the  other  is  completely  empty.  It  will 
be  seen  from  Fig.  63  (5),  that,  considering  the  armature 
to  be  revolving  at  a  speed  of  10  revolutions  per  second, 
then  in  the  ^J^th  part  of  a  second,  the  loop  c  D  E  F  will 
be  advanced  to  the  position  c'  D'  E'  F',  represented  by  the 
dotted  line,  and  the  amount  of  flux  then  emptied  out  of 
it  will  be  that  included  in  the  two  narrow  parallelograms 
c  c'f'fsmdd  d'  e'  e,  which  are  included  between  the  pro- 
jections of  the  sides  of  the  loop  in  the  two  positions.  If 


131 


the  density  of  the  flux  be  three  kilogausses  and  the  areaof 
the  two  parallelograms  34  square  cms.,  the  flux  emptied  out 
in  this  yJrj-th  part  of  a  second  will  be  3,000  X  34  =  102 

kilowebers,  representing  a  rate  of  filling  of  - 

240 

24,480,000  webers  per  second  —  0.2448  volts  generated 
in  the  loop. 

On  the  contrary,  the  horizontal  loop  G  H  j  K,  repre- 
sented separately  in  Fig.  63  (c\  will  have  advanced  dur- 
ing the  same  -^th  of  a  second,  to  the  position  repre- 
sented by  the  dotted  line  G'  H'  j'  K',  and  will  only  have 


a.  i 

FIG.  63. 

introduced  into  it  the  flux  included  in  the  parallelogram 
g'  h'  j'  k'.  With  the  same  density  of  flux  the  area  of 
this  parallelogram  will  be  259  square  cms.,  making  a 
total  loss  of  flux  in  the  ^^th  of  a  second  =  777,000 
webers,  and  the  rate  of  emptying  777,000  X  240  webers 
per  second  =  186,480,000,  an  E.  M.  F.  in  the  direction 
shown  by  the  arrows  of  1.8648  volts.  It  is  easy  to  see 
that  if  the  interval  of  time  had  been  taken  sufficiently 
small,  the  E.  M.  F.  in  the  horizontal  loop  would  amount  to 
1.885  volts,  while  in  the  vertical  loop  it  would  be  zero. 
For  loops  that  may  lie  upon  the  surface  of  the  armature, 
between  the  vertical  and  horizontal  positions,  the  value 
of  the  E.  M.  F.  generated  will  be  intermediate  between 
zero  and  1.885. 


132 


The  rule  for  determining  the  direction  of 
E.  M.  F.  induced  in  a  loop,  is  as  follows.  When  a 
watch  is  held  in  front  of  an  observer,  the  light  by  which 
he  sees  the  dial,  passes  directly  from  the  dial  to  his  eye. 
If  now  the  watch-face  be  regarded  as  a  loop,  then  when 
flux  is  poured  through  it  in  the  same  direction  as  the 
light  (i.e.,  entering  at  the  back  and  passing  towards  the 
observer),  the  E.  M.  F.  around  the  loop  will  be  in  the  same 
direction  as  the  motion  of  the  hands  of  the  watch.  If, 
on  the  contrary,  the  motion  of  the  flux  be  against  the 
motion  of  the  light  the  direction  of  the  E.  M.  F.  will  be 
against  the  direction  of  the  hands  of  the  watch. 

Decreasing,  or  pouring-out  flux,  produces  an  E.  M.  F. 
in  the  opposite  direction  to  increasing  or  pouring-in 
flux.  Thus,  if  the  loop  c  D  E  F,  Fig.  63  (a\  be  rotated 
about  its  axis  A  B,  in  the  flux  indicated  by  the  arrows, 
into  the  position  represented  by  the  dotted  lines,  it  will 
be  pouring  flux  out,  equivalent  to  having  flux  poured 
into  it  from  the  opposite  side,  and  the  E.  M.  F.  induced 
around  the  loop,  during  the  passage  shown,  will  be  in 
the  direction  of  the  arrows.  The  application  of  this 
rule  to  the  loop,  shown  in  Fig.  63  (c\  will  show  that  the 
rotation  of  the  loop,  in  the  direction  indicated  by  the 
arrow,  will  produce  an  E.  M.  F.  which  will  be  directed 
during  the  motion  of  the  loop  from  G  to  H,  and  from  j  to  K. 

142.  A  diagram  representing  a  ring-wound  armature, 
commonly  called  a  Gramme-ring  armature,  from 
the  name  of  the  French  electrician,  Gramme,  who  first 
employed  it  in  practice,  is  shown  in  Fig.  64  (a). 

The  flux  is  supposed  to  enter  the  ring  on  the  side 
B  c  D  E  F,  and  to  leave  it  at  the  side  M  L  K  j  H. 

It  will  be  observed  that  the  flux  fills  the  loops  at  the 


133 


top  and  bottom  of  the  ring,  while  the  loops  on  the  hori- 
zontal line  are  empty ;  the  intermediate  loops,  having 
intermediate  quantities  of  flux  passing  through  them.  As 
before,  the  E.  M.  F.  in  the  horizontal  loops  will  be  a 
maximum,  since,  at  this  position,  the  rate  of  filling  is  a 
maximum,  and  the  loops  on  the  vertical  line  have  no 
E.  M.  F.  in  them. 

143.     Fig.  64  (5),  shows  the  connection  of  the  loops  on 
the  Gramme  ring  in  a  single  series  to  form  a  com- 
plete circuit.      When  the  ring   is   set  in  rotation,  the 


FIG.  64. 

E.  M.  F.  in  the  various  turns  unite  on  one  side  of  the 
vertical  line  to  produce  a  total  E.  M.  F.  which  is  equal 
and  opposite  to  that  produced  by  the  loops  connected  in 
series  on  the  other  side  of  the  vertical  line,  so  that,  pro- 
vided the  winding  be  uniform,  and  riot  dissymmetrical, 
no  current  will  be  produced  in  the  ring,  the  two  oppos- 
ing E.  M.  F.'S  balancing,  for  otherwise  wasteful  currents 
would  probably  be  set  up  in  the  armature.  If  two 
brushes,  z,  z',  were  placed  at  points  on  the  vertical  line, 
so  as  to  maintain  contact  with  the  armature  wires  during 
its  rotation  as  they  come  round,  then  the  E.  M.  F.'S  gene- 
rated in  the  opposite  sides  of  the  armature  would  tend 


134 


to  pour  a  continuous  current  through  the  circuit  con- 
nected with  the  brushes.  In  practice,  since  the  armature 
is  wound  with  insulated  wire,  often  laid  on  in  several 
layers,  it  is  found  convenient  to  carry  out  connections  at 
regular  intervals  to  insulated  conducting  segments  of  a 
device  called  a  commutator.  The  brushes  are  rested  on 
the  surface  of  the  commutator  at  the  proper  points. 
The  voltaic  analogue  of  the  E.  M.  F.'S  in  the  armature  are 
shown  in  Fig.  64  (c). 

144.  The  amount  of  E.  M.  F.  produced  in  any  case 
may  be  determined  by  the  following  rule.     Mul- 
tiply the  total  flux  in  webers,  passing  through  each  pole 
into  the  armature,  by  the  number  of  revolutions  of  the 
armature  per  second,  and  by  the  number  of  wires  counted 
once  round  the  surface  of  the  armature.     The  quotient 
divided  by  100,000,000  will  give  the  volts.     The  total 
flux  may  be  determined  when  the  total  reluctance  of  the 
magnetic  circuit,  or  circuits  and  the  M.  M.  F.  of  the  field 
magnets  are  known. 

145.  The  output  of  a  dynamo  machine  is  most  con- 
veniently given  in  kilowatts,  and  is    found   by 

multiplying  the  pressure  in  volts  which  the  machine 
sustains  at  its  terminals,  by  the  current  in  amperes  it 
maintains  at  full  load.  Thus,  a  railway  generator 
producing  a  current  of  952.4  amperes  at  525  volts  ter- 
minal pressure,  will  develop  in  the  external  circuit  an 
activity  of  952.4  X  525  =  500,000  watts,  and  the  ma- 
chine will  be  a  500  K.  w.  machine.  In  practice  a  certain 
relation  always  exists  between  the  output  of  a  machine, 
its  E.  M.  F.,  and  internal  resistance.  It  is  evident  that  if 
the  resistance  of  the  machine  exceeds  a  certain  value,  the 


current  passing  through  that  resistance  at  a  continuous 
K.  M.  F.  and  output,  would  produce  an  excessive  and  dan- 
gerous amount  of  heat  in  the  machine.  Indeed,  in  prac- 
tice, the  only  electrical  difference  existing  between  a 
3,000  K.  w.  machine,  say,  of  500  volts,  and  a  one  K.  w. 
machine,  at  the  same  pressure,  lies  in  the  resistance  of 
its  armature. 

146.  Were    it  practicable  to  operate  a  dynamo  on 
short  circuit,  the  maximum  possible  activity  would 

T?       TT* 

be  obtained,  and  would  be  equal  to  J?/  =  J^X  —  =  — 

r        r 

watts,  where  E,  is  the  E.  M.  F.  of  the  machine  in  volte, 
and  /*,  its  internal  resistance  in  ohms.  This  theoretical 
maximum  output  may  be  called  the  electrical  capability 
of  the  machine,  and,  in  practice,  a  certain  fraction  of 
this  electrical  capability  may  be  taken  as  the  output. 
This  fraction  varies  with  the  character  and  size  of  the 
machine,  from  0.1  in  small  machines  to,  say,  0.02  in 
machines  of  200  K,  w. 

147.  The  electrical  capability  of  a  machine  is  not 
altered  by  the  size  of  the  wire  employed  in  the 

winding,  provided  the  volume  of  the  winding  space  on 
the  armature  be  maintained  constant,  and  that  the  pro- 
portion of  space  allotted  to  insulation  remains  constant. 
Thus,  if  the  diameter  of  the  wire  employed  be  halved, 
there  will  be  room  for  four  times  as  many  wires,  and  the 
total  resistance  of  the  armature  will  be  increased  16 
times,  since  the  length  has  been  quadrupled,  but  the 
cross-section  of  each  turn  reduced  to  one-fourth.  The 
ratio  of  E*  to  r,  will  be  ^| ;  'i.e.9  will  remain  the  same. 
This  is  equivalent  to  the  statement,  that,  provided  the 


136 


insulation  space  of  the  armature  remains  constant,  the 
output  of  the  machine  remains  the  same  at  any  voltage, 
so  that  if  the  same  machine  be  wound  for  30  or  60  or 
100  volts,  its  output  will  remain  unchanged. 

SYLLABUS. 

When  the  flux  is  being  poured  into  a  loop  in  the  same 
direction  as  the  light  passing  from  a  watch  face  towards 
the  observer,  the  direction  of  the  induced  E.  M.  F.  is  the 
same  as  the  motion  of  the  hands  of  a  watch. 

The  electrical  capability  of  a  machine  is  equal  to 
square  of  its  E.  M.  F.  in  volts,  divided  by  its  resistance  in 
ohms.  The  electrical  capability  bears  a  ratio  to  the  out- 
put varying  according  to  the  type  and  size  of  the  ma- 
chine. This  ratio  is  within  wide  limits  independent  of 
the  E.  M.  F.  for  which  the  machine  is  wound. 

Laboratory  of  Houston  &  Kennelly, 
Philadelphia. 


[Copyright,  1894,  by  THE  ELECTRICAL  ENGINEER.") 


WEEKLY. 


No    1  8  OPTOT*™  131  8Q4-         Price»     "    10  Cents- 

Ld,  1    te.        Subscription,  $3.00. 

Electrical   Engineering  Leaflets, 


Prof.  E.  J.  Houston,  Ph.  D. 

AND 

A.  E.  Kennelly,  F.  R.  A.  S. 


INTERMEDIATE  CF?ADE 

DYNAMO. 


148.  By  the  electrical  efficiency  of  a  dynamo  is 
meant  the  ratio  between  the  electrical  activity  in 
the  external  circuit  of  the  machine,  and  the  total  elec- 
trical activity  it  produces  both  in  its  internal  and  exter- 
nal circuits.  Thus,  if  a  dynamo  develops  an  activity  of 
100  kilowatts  in  its  external  circuit  (say,  1,000  amperes 
at  100  volts,  as  measured  at  its  terminals),  and  expends, 
electrically,  four  kilowatts  in  its  armature  and  field  mag. 
nets,  i.e.,  internally,  then  the  total  electrical  activity  in 
the  circuit  will  be  104  K.  w.,  and  the  electrical  efficiency 
of  the  machine  will  be  expressed  by 

External  Activity __  100 

Internal  Activity  +  External  Activity       104 

The  commercial  efficiency  of  the  machine  differs  from 
this,  and  is  the  ratio  existing  between  the  output  of  the 
machine  and  its  intake.  Thus,  if  the  same  machine  ex- 
pended, besides  the  four  KW.  in  its  field  and  armature, 
say,  five  KW.  in  mechanical  friction  and  other  losses, 

Published  by 

THE   ELECTRICAL  ENGINEER, 
203  Broadway,  New  York  N.  Y. 

[Entered  as  second-class  matter  at  the  New  York,  N.  Y.,  Post  Office,  June  14,  1894.] 


138 


then  the  total  activity  expended  in  the  machine  will  be 
nine  KW.,  and  its  commercial  efficiency  will  be 
External  Activity  or  Output  _  100  _  ~  Q-^A 
Intake  ~  109  ~ 

149.  In  any  generator  the  following  losses  prevent 
the   output  from   being    equal    to   the    intake ; 

namely. 

(1.)  Mechanical  losses,  including  friction  of  all  kinds. 

(2.)  Electrical  losses.  These  are  of  two  kinds ;  that 
in  the  conducting  circuit  on  the  armature  due  to  the 
passage  of  an  outgoing  current  through  the  resistance 
of  the  armature,  and  that  due  to  small  local  or  eddy 
currents  in  the  substance  of  the  wire  on  the  armature, 
or  in  the  iron  of  the  armature  core  and  pole-pieces. 

(3.)  Magnetic  losses,  or  those  due  to  the  reversal  of 
the  magnetism  in  the  iron. 

150.  The  mechanical  losses  may  be  classed  as  follows, 
viz.,  air  friction,  brush  friction  and  journal  fric- 
tion. 

If  the  armature  resistance  of  a  100  KW.  dynamo  be 
0.005  ohm,  and  the  current  it  delivers  800  amperes,  the 
activity  expended  in  heating  the  armature  wire  will  be 
#r=  800  X  800  X  0.005  =  3,200  watts ;  i.e.,  3,200 
joules  per  second. 

The  loss  in  the  armature  winding  of  a  generator  of 
one  KW.  capacity  is  often  12  per  cent,  of  the  output, 
while  in  the  armature  winding  of  a  200  KW.  generator 
it  is  usually  about  two  per  cent,  of  the  output. 

Similarly, .  if  the  field  magnets  require  a  current  of 
eight  amperes  to  excite  them  and  are  supplied  direct 
from  the  brushes,  the  energy  expended  in  heating  their 


139 


circuit  will  be  E  I  =  125  X  8  =  1,000  watts.  This 
amount  will  vary  considerably  with  the  type  and  size  of 
machine,  say,  from  10  per  cent,  of  the  output  in  a  2  KW. 
generator  to  1.5  per  cent,  of  the  output  in  a  200  KW. 
generator.  These  losses  constitute  the  electrical  losses 
in  the  circuits  of  the  machine. 

The  rapid  reversals  of  magnetism  to  which  the  iron  in 
the  armature  and  in  the  pole-pieces  is  subjected,  during 
the  operation  of  the  machine,  set  up  E.  M.  F.'S  in  these 
masses  which  in  their  turn  produce  local,  deleterious 
currents  in  the  iron,  called  eddy  currents.  Although 
the  E.  M.  F.  producing  these  currents  may  be  only 
a  small  fraction  of  a  volt,  yet  the  resistance  of  the  mass 
of  metal  in  which  they  are  set  up,  being  also  very  small, 
.the  actual  currents  produced  may  be  seriously  large; 
hence  it  is  necessary  to  check  the  establishment  of  these 
wasteful  currents  by  limiting  the  mass  of  metal  in  which 
they  can  exist  as  a  single  circuit.  This  is  accomplished, 
in  practice,  by  laminating  the  iron  in  a  direction  parallel 
to  the  direction  of  the  magnetic  flux.  It  is  not  neces- 
sary elaborately  to  insulate  the  separate  laminae  or  sheets 
of  iron  so  employed,  since  the  film  of  oxide  always  pre- 
sent on  their  surfaces  is  sufficient  to  prevent  the  feeble 
E.  M.  F.'S  from  crossing  them.  Where,  however,  great 
mechanical  pressure  is  brought  to  bear  upon  such  surfaces 
during  construction,  they  are  generally  insulated,  either 
by  dipping  them  in  shellac  varnish,  or  by  interposing 
sheets  of  tissue  paper. 

151.     Similar  eddy  currents  are  also   set  up   in  the 

substance  of  the  copper  wire  on  the  armature ; 

and,  when  such  conductors  are  of  large  cross-section,  it  is 

necessary  to  subdivide  that  cross-section  by  transposing 


140 


and  stranding  the  conductors;  i.e.,  by  dividing  them  into 
a  number  of  separate  conductors.  If,  however,  the  wire 
be  wound  in  grooves  on  the  armature  core,  as  in  the 
case  of  toothed  armatures,  or  armatures  having  iron  pro- 
jections, lamination  of  the  conductors  is  rendered  un- 
necessary, since  the  copper  is  practically  insulated  from 
the  flux  which  links  with  it,  and  which  passes  almost  en- 
tirely through  the  iron  teeth.  All  electrical  losses  of 
the  character  of  eddy  currents  belong  to  the  P  R  type, 
and,  since  f,  increases  with  the  E.  M.  F.,  which  in  its  turn 
increases  with  the  speed,  such  losses  increase  as  the  square 
of  the  speed  of  rotation.  If,  therefore,  the  losses  due 
to  eddy  currents  in  a  given  machine  are  300  watts,  at  its 
normal  speed,  they  would  amount  to  1,200  watts,  if  the 
speed  were  doubled. 

152.  The  magnetic  losses,  which  occur  in  the  iron  of 
the  armature  core,  are  due  to  what  is  called  mag- 
netic hysteresis  (his-ter-ee'-sis).  The  word  hysteresis,  de- 
rived from  the  Greek,  means  a  lagging  behind,  thus 
characterizing  the  lagging  of  the  magnetization  in  the 
iron,  or  other  magnetic  metal,  behind  the  magnetizing 
flux.  That  is  to  say,  when  a  magnetizing  flux  is  brought 
to  bear  upon  a  piece  of  iron,  the  molecules  of  the  iron 
become  aligned,  as  already  explained.  On  the  with- 
drawal of  the  magnetizing  flux,  the  bar  does  not  in- 
stantly lose  its  magnetism ;  i.e.,  its  alignment,  but  tends 
to  retain  the  same  for  a  short  time,  and  does  not  reach  a 
condition  of  demagnetization  until  the  flux  has  not  only 
disappeared  but  has  actually  been  reversed.  In  other 
words,  the  magnetic  flux  in  the  iron  lags  behind  the 
magnetizing  flux. 


141 


153.  When  a  magnetic  flux  is  produced  in  air  around 
a  conductor,  energy  is  absorbed  into  the  ether  and 
air  from  the  circuit ;  but,  on  the  cessation  of  the  current, 
all  this  energy  is  returned  to  the  circuit  electrically.  If, 
however,  iron  be  magnetized  by  the  current,  energy  will 
be  absorbed  from  the  circuit,  both  by  the  ether  and  by 
the  iron,  with  this  difference,  that  while  the  energy  in 
the  ether  will  be  restored  to  the  circuit  as  electrical  en- 
ergy, on  the  withdrawal  of  the  magnetizing  flux,  that  in 
the  iron  will  be  only  partially  restored  to  the  circuit,  as 
electrical  energy,  the  balance  being  expended  in  the  iron 
as  heat.  Since  the  same  amount  of  heat  is  produced  at 
each  cycle,  at  every  reversal,  if  the  iron  be  carried 
through  50  cycles  (50  double  reversals  of  magnetism  per 
second),  there  will  be  50  times  as  much  energy  expended 
in  a  cubic  centimetre,  or  cubic  inch,  as  if  only  a  single 
cycle  were  made  per  second.  As  the  limiting  flux  den- 
sity, through  which  the  iron  is  carried  in  each  cycle,  is 
increased,  the  hysteretic  loss  increases  in  greater  ratio, 
and  a  doubled  range  of  flux  density  is  accompanied  by 
approximately  a  trebled  loss  of  energy  in  hysteresis. 
Thus,  if  an  iron  armature  be  revolved  in  a  bipolar  mag- 
netic field  20  times  a  second,  every  cubic  inch  of  iron 
will  be  magnetized  from,  say,  a  flux  density  of  5,000 
gausses  in  one  direction,  to  5,000  gausses  in  the  opposite 
direction,  in  20  complete  cycles  or  double  reversals 
per  second,  and  every  cubic  inch  of  such  iron  will  have 
expended  in  it  0.0543  joules  per  second.  If  now  the 
field  magnets  be  excited  by  an  increased  current,  so 
as  to  bring  the  flux  density  in  the  armature  up  to  10,000 
gausses,  the  range  of  reversal  will  be  doubled,  and  the 
hysteretic  loss  in  every  cubic  inch  will  be  practically 


142 


trebled ;  i.e.,  increased  to  0.165  joule  per  second,  or  an 
activity  of  loss  of  0.165  watt. 


(GAUSSES)  DENSITY 


FIG.  65. 

Hysteretic  Diagrams  of  Charcoal  Iron  Rings  and  of  Hard  Cast  Steel. 

Charcoal  Iron -.-Full  line  to  indicated  scale.  From  observations  of  Kennelly. 
JC  ±  6,  (B  ±  10,600. 

Hard  Cast  Steel :— Broken  line,  to  10  times  indicated  scale.  From  observations  of 
Steinmetz.  3C  ±  82,  (B  ±  11,500. 


143 


154.  Fig.  65  represents  what  is  called  a  liysteretic 
diagram  or  cycle,  and  shows  how  the  flux  density 
in  iron  varies  with  the  cyclic  variation  of  the  magnetiz- 
ing flux.  Thus  the  prime  intensity  or  magnetizing  flux 
which  will  bring  this  sample  of  iron  to  an  intensity  of 
(B  =  10,600  gausses  is  6.1  gausses;  carrying  back  the  mag- 
netizing force,  i.  e.,  reversing  it  from  3C  back  to  zero, 
the  intensity  in  the  iron  does  not  return  along  the  same 
path  A  B  c,  it  took  during  ascension,  but  descends  along 
the  more  slowly  returning  curve  ODE,  and,  when  the 
magnetizing  flux  reaches  zero;  i.e.,  when  the  magnetiz- 
ing flux  is  completely  withdrawn,  there  is  still  a  residual 
magnetic  flux  of  (B  =  8,900  gausses  in  the  iron.  In  fact, 
the  magnetic  flux  has  to  be  carried  back  to  z  or  —  2.2  gaus- 
ses, in  order  to  destroy  the  magnetic  flux  in  the  iron,  i.  <?., 
to  reduce  it  to  zero.  This  negative  magnetizing  flux  0  z, 
in  gausses,  is  the  measure  of  the  hardness  of  the  sample 
of  iron.  Yery  soft  iron  will  take  a  small  negative  0  z, 
to  destroy  its  flux,  while  hard  steel  requires  a  powerful 
0  z.  In  fact,  0  z,  is  the  measure  of  the  coercive  forces 
or  retentivity  in  the  iron.  Continuing  the  magnetizing 
flux  to  —  6.1  gausses,  the  flux  density  in  the  iron  descends 
to  — 10,600  gausses,  or  becomes  equal  in  intensity  to  its 
value  at  c,  but  in  the  opposite  direction,  and  the  cycle  is 
completed  along  the  line  H  j  K  L  c,  by  reversing  the 
magnetizing  flux  from  negative  to  positive.  The  area 
of  this  loop  is  a  measure  of  the  loss  of  energy  in  the 
iron.  The  broken  line  cycle  represents  a  corresponding 
diagram  for  hard  steel,  drawn  to  one-tenth  scale  in  mag- 
netizing flux.  It  will  be  seen  for  the  same  range  of  flux 
density  in  steel  that  the  area  of  the  loop  inclosed  would 
be  about  ten  times  greater,  if  drawn  to  the  same  scale. 


144 


If  these  various  losses  are  summed  and  deducted  from 
the  intake  of  the  generator,  the  balance  will  be  the  out- 
put of  the  machine,  and  the  ratio  of  this  output  to  the 
intake  will  give  the  commercial  efficiency. 

SYLLABUS. 

The  electrical  efficiency  of  a  generator  is  the  ratio  of 
the  external  activity  to  the  total  electrical  activity. 

The  commercial  efficiency  is  the  ratio  of  the  output  or 
external  activity,  to  the  intake. 

The  output  of  the  machine  is  lower  than  the  intake 
on  account  of  losses  arising  from  mechanical  friction, 
electrical  friction,  and  magnetic  friction. 

Hysteresis  is  the  lagging  of  the  magnetization  in  a 
magnetic  metal  behind  the  magnetizing  flux. 

Eddy  current  losses  in  a  dynamo-electric  machine  in- 
crease with  the  square  of  the  speed  of  rotation. 

Hysteretic  loss  increases  directly  with  the  speed  of 
rotation. 

Laboratory  of  Houston  &  Kennelly, 
Philadelphia. 


[Copyright,  1894,  by  THE  ELECTRICAL  ENGINEER. 
WEEKLY. 

No.  19.  OCTOBER  20,  1894. 

Electrical   Engineering  Leaflets, 


Prof.  E.  J.  Houston,  Ph.  D. 

AND 

A.  E.  Kennelly,  F.  R.  A.  S. 


INTERMEDIATE 

DYNAMO 


155.  In  practice   it  is  impossible  to  obtain  from  a 
generator  the  maximum  output  which  it  is  capa- 
ble  of   producing,   since   at   a   certain   critical   output, 
varying  with  the  type  and  character  of  the  machine,  a 
limitation  is  reached  due  to  one  or  more  of  three  con- 
siderations ;  namely, 

(1.)  A  limitation  due  to  excessive  drop  in  the  arma- 
ture, an  insufficient  E.  M.  F.  remaining  at  the  termminals. 

(2.)  A  limitation  due  to  overheating  of  the  machine. 

(3.)  A  limitation  due  to  dangerous  sparking  at  the 
commutator. 

156.  A  generator  is  capable  of  producing  a  certain 
maximum  E.  M.  F.     The  drop  of  pressure    due 

to  1  J?,  increases  with  the  load,  principally  owing  to 
the  increase  in  /,  the  current  strength  delivered,  and 
partly  because,  as  the  temperature  of  the  machine  in- 
creases, the  value  of  J?,  increases  by  about  J  per  cent, 
per  degree  Fahrenheit.  If  the  drop  becomes  excessive, 


Published  by 

THE   ELECTRICAL  ENGINEER, 
ao3  Broadway,  New  York  N.  Y. 

[Entered  as  second-class  matter  at  the  New  York,  N.  Y.,  Post  Office,  June  14,  1894.] 


146 


the  E.  M.  F.  remaining  at  the  terminals  may  be  insuffi- 
cient to  deliver  the  pressure  required.  Thus,  if  a  gen- 
erator, connected  to  an  incandescent  light  circuit,  has  a 
maximum  E.  M.  F.  of  125  volts,  and  the  drop  in  the  ma- 
chine be  eight  volts,  the  pressure  at  maximum  E.  M.  F. 
will  be  117  volts,  which  will  be  insufficient  to  bring  the 
lamps  up  to  candle-power  if  they  be  made  for  120  volts 
pressure.  The  output  will,  therefore,  have  to  be  re- 
duced in  order  to  bring  the  lamps  up  to  candle-power. 
When,  however,  a  dynamo  has  been  properly  installed 
with  a  view  to  the  work  it  has  to  perform,  its  limitation 
to  load  is  not  usually  want  of  pressure.  Such  a  limita- 
tion is  more  readily  encountered  in  small  generators  than 
in  large  ones. 

157.  In  the  practical  operation  of  generators,  the  prin- 
cipal limitation  of  output  is  that  due  to  excessive 
heating.  Although  heat  is  produced  in  both  the  field  and 
armature,  through  the  influences  of  the  losses  of  energy 
taking  place  in  the  machine,  yet  the  principal  elevation 
of  temperature  is  usually  found  to  be  in  the  armature. 

Were  it  possible  to  construct  an  armature  of  a  dynamo  of 
iron  and  copper  only,  that  is,  without  any  solid  insulating 
material,  the  only  objection  that  would  exist  to  operating 
the  generator  at  such  an  output  as  would  produce  a  high 
temperature,  say  300°  C.  in  the  armature,  would  be  the 
commercial  value  of  the  energy  expended  in  the  resis- 
tance of  the  machine  at  this  temperature.  Since,  how- 
ever, insulating  material,  of  a  character  readily  dam- 
aged by  excessive  heating,  has  to  be  employed,  the 
critical  temperature  beyond  which  it  is  undesirable  to 
operate  a  generator  is  far  lower,  say  100°  C.  This 
maximum  temperature  of  100°  C.  would  represent  a 


147 


temperature  elevation  of  75°  C.  above  the  normal  tem- 
perature of  the  air,  assumed  at  25°  C.  But  the  output, 
which  would  permit  of  such  an  elevation  of  temperature, 
would  not  be  possible  in  cases  where  the  surrounding 
temperature  happened  to  be  35°  C.,  and  would  pre- 
vent any  allowance  for  accidental  overload.  Taking 
both  these  liabilities  into  account,  it  is  found  desirable  in 
practice  to  limit  the  temperature  elevation  of  a  gener- 
ator to  50°  C.  above  surrounding  objects,  representing 
in  the  case  of  a  normal  temperature  of  35°  C.,  a  tempera- 
ture attained  of  85°  C.,  and  allowing  even  then  a  margin 
for  accidental  overload  without  endangering  the  insula- 
tion of  the  machine.  Conservative  practice  is,  however, 
reducing  this  heat  elevation  to  40°  C. 

158.  A  machine  with  a  very  low  temperature  eleva- 
tion is  a  machine  with  a  large  reserve  of  output, 
unless  that  reserve  be  annulled  by  a  tendency  to  spark- 
ing or  by  excessive  drop  of  pressure.  When  a  generator 
is  started  at  full  load,  its  rate  of  increase  of  temperature 
is  a  maximum  and  diminishes  as  time  goes  on,  owing  to 
the  fact  that  it  tends  to  attain  its  ultimate  condition,  in 
which  the  loss  of  heat  from  the  armature  is  exactly  equal 
to  the  rate  of  generation  of  heat  within  it. 

The  heat  generated  in  the  armature  is  dissipated  from 
its  surface  by  conduction,  radiation,  and  convection. 
The  velocity  with  which  these  influences  enable  the  heat 
to  be  carried  away,  varies  with  the  dimensions,  speed,  and 
type  of  machine,  but  it  is  usual  to  allow  a  certain  amount 
of  surface  to  the  armature  for  a  given  known  amount  of 
energy  expended  within  it;  for  ordinary  drum-wound 
armatures  this  allowance  is  usually,  0.15  watt  per  sq. 
em.,  (approximately  one  watt  per  sq.  inch.) 


rV   OF 

UNIVERSITY: 


148 


In  specially  ventilated,  hollow  armatures  this  allowance 
can  sometimes  be  increased  to  0.45  watt  per  square  cen- 
timetre ;  (approximately  three  watts  per  square  inch.) 

159.  Generators  are  usually  guaranteed  not  to  elevrate 
their  temperatures  at  any  part  more  than  a  certain 

limit  above  the  surrounding  air,  during  a  prolonged  run 
at  full  load.  The  instrument  ordinarily  employed  to 
measure  the  elevation  of  temperature  is  a  naked  ther- 
mometer placed  on  some  part  of  the  machine  and  cov- 
ered by  some  thermal  non-conducting  material,  such  as 
cotton-waste.  In  order  to  apply  the  thermometer  to  the 
armature,  the  machine  has  to  be  stopped  and  a  certain 
time  has  to  elapse  before  the  thermometer  applied  to  the 
surface  of  the  armature  can  attain  its  maximum  read- 
ing, since  the  the  interior  of  the  armature  will  have  a 
higher  temperature  than  that  on  the  surface  and  time  is 
required  for  the  maximum  temperature  of  the  surface  to 
be  reached. 

160.  The   remaining   limitation   of  output,  namely, 
that  due  to  dangerous  sparking  at  the  brushes,  is 

usually  reached  in  well-designed  machines  at  outputs 
greater  than  those  of  temperature  elevation.  The  spark- 
ing, which  always  occurs  at  continuous-current  dynamo 
brushes  in  a  greater  or  less  degree,  is  due  to  the  effect  of 
inductance  in  the  coil  which  the  brush  is  breaking  con- 
tact with  at  the  commutator,  owing  to  the  E.  M.  r..  which 
is  developed  in  that  coil  by  the  sudden  reversal  of  its 
current. 

Thus,  in  Fig.  66,  which  represents  diagramatically  a 
portion  of  a  Gramme-ring  armature,  the  brush  H,  is  rest- 
ing on  the  commutator  bar  J,  connected  with  the  coil  B, 


149 


the  directions  of  the  current  in  the  winding  being  indi- 
cated by  the  small  arrows,  and  the  direction  of  motion  of 
the  armature  indicated  by  the  large  arrow.  Since  the 
effect  produced  is  one  of  relative  motion,  the  rotation  of 
the  armature,  in  the  direction  of  the  arrow  with  the  brush 
at  rest,  will  be  equivalent  to  the  rotation  of  the  brush 
over  the  commutator  in  the  opposite  direction  to  the 
arrow  with  the  armature  remaining  at  rest.  In  this  way 
we  may  suppose  the  brush  carried  from  H  to  h.  In  so 


FIG.  66. 

Commutation  of  segments  on  the  

Gramme-ring  armature. 

FIG.  67. 

doing  it  will  iirst  short-circuit  the  coil  B,  connected 
with  the  segments  5  and  c.  If  no  current  existed  in  the 
armature,  i.  <?.,  if  the  external  circuit  were  open,  and  the 
brush  is  in  the  right  position,  there  would  be  no  current 
in  the  coil  B,  during  short-circuiting ;  but,  since  when 
the  load  current  is  flowing  through  the  armature  all  the 
coils  on  the  left  hand  side  of  the  brush  have  currents 
flowing  through  them  downwards,  as  indicated  by  the 
arrows,  and  all  on  the  right  hand  side  upwards,  as  similarly 


150 


indicated,  a  reversal  of  the  current  in  each  coil  must 
take  place  during  the  period  of  short-circuiting.  If  the 
reversal  has  not  completely  taken  place  during  this 
period,  there  will  be  a  tendency  for  a  spark  to  follow  the 
brush  from  the  segment  J,  when  the  brush  is  transferred 
from  b  to  tf,  owing  to  the  E.  M.  F.  in  the  coil,  set  up  by 
the  sudden  reversal  of  its  current.  When,  therefore,  the 
load  current  is  strong  in  B,  it  is  necessary  to  employ  a 
counter  E.  M.  r  in  B,  for  the  purpose  of  reversing  its  cur- 
rent during  the  period  of  short-circuiting.  This  is  ac- 
complished by  giving  the  brushes  a  lead,  or  a  movement 
forward  in  the  direction  in  which  the  armature  is  rotat- 
ing, in  order  to  bring  the  coil  under  reversal  into  flux 
from  the  field  magnets,  the  direction  of  which  is  calcu- 
lated to  reverse  the  load  current  in  B,  by  its  movement 
during  the  period  of  short-circuit. 

161.  As  the  current  load  increases,  to  prevent  spark- 
ing, this  lead  of  the  brushes  has  to  be  increased  in 
order  to  bring  the  coil  into  stronger  flux.  As  soon,  how- 
ever, as  a  certain  current  strength  is  reached,  no  increase  in 
the  lead  will  have  any  effect  in  diminishing  the  spark- 
ing. The  reason  for  this  will  be  seen  from  an  inspection 
of  Fig.  67,  which  diagrammatically  represents  a  four- 
pole  generator  in  which  2  z,  and  2'  z' ,  respectively,  repre- 
sent tlu  diameters  of  commutation,  or  the  position  at 
which  the  brushes  should  be  applied  to  the  armature,  in 
order  to  carry  off  the  currents.  When  current  is  taken 
from  the  armature,  the  brushes  require  to  be  shifted 
into  positions  such  as  b  and  V  ;  i.e.9  given  a  lead  in  order 
to  prevent  sparking.  At  the  quadrant  A,  the  flux  is  re- 
presented as  passing  from  pole  to  armature  under  normal 
circumstances  with  no  M.  M.  F.  in  the  armature  and  full 


151 


M.  M.  F.  in  the  field.  At  quadrant  B,  the  flux  is  indicated 
as  it  may  be  produced  under  M.  M.  F.,  from  the  armature 
with  a  strong  load  current  through  its  windings,  and  no 
M.  M.  F.  in  the  field,  and  represents  a  condition  of  arma- 
ture excitation  taking  place  under  the  pole  A,  when  the 
load  current  flows  through  the  armature. 

At  quadrant  #,  these  two  effects  are  superposed,  and  a 
distortion  results  in  the  distribution  of  field  flux,  as 
shown,  whereby  the  intensity  in  the  air  and  armature 
are  increased  at  the  edge  of  the  pole  6,  and  decreased 


EUc.Engineer 

FIG.  68. 

Section  of  one  Quadrant  of  a  4-pole  Generator  with  Tooth-cored  Armature. 

at  the  edge  5.  This  distortion  due  to  armature  reac- 
tion weakens  the  controlling  flux  at  the  position  where 
commutation  is  taking  place,  and  towards  which  the  brush 
has  been  moved.  It  is,  therefore,  necessary  to  still  further 
advance  the  brush  in  order  to  bring  the  commutated  coils 
into  a  stronger  flux.  A  load  current  will  finally  be  reached 
at  which  it  is  impossible  to  obtain  the  controlling  flux  near 
the  position  of  commutation,  and,  at  such  a  current 
strength,  no  amount  of  lead  in  the  brushes  avails  for 
checking  sparking.  This  current  is  beyond  the  sparking 
limitation  of  the  machine. 


152 


162.  Toothed  armatures,  however,  such  as  shown  in 
Fig.  68,  if  properly  designed,  can  be  made  to 
carry  a  stronger  current  without  sparking  than  smooth- 
core  armatures ;  for  the  tendency  to  crowd  the  flux  at 
the  leading  corner  of  the  pole  pieces,  and  denude  it  at 
the  trailing  corner,  is  opposed  by  the  increasing  reluc- 
tance which  the  iron  teeth  can  be  caused  to  exert  to- 
wards such  increase  of  intensity,  if  they  are  brought 
sufficiently  near  to  such  reluctance  under  the  ordinary 
conditions  of  load. 

SYLLABUS. 

The  limitations  to  the  output  of  a  dynamo  are  of  three 
types ;  namely, 

(1.)  Those  due  to  fall  of  pressure  in  the  machine. 

(2.)  Those  due  to  excessive  heating  in  the  machine. 

(3.)  Those  due  to  dangerous  sparking  at  the  brushes. 

Limitations  arising  from  drop  are  reached  when  the 
maximum  pressure,  available  at  the  armature  terminals, 
falls  below  that  required  by  the  circuit. 

The  limitations  due  to  heating  of  the  machine  are  im- 
posed for  insuring  the  safety  of  the  insulation  of  the 
conductor  on  the  machine. 

The  limitations  due  to  sparking  are  imposed  for  insur- 
ing the  proper  operation  and  durability  of  the  commu- 
tator. 

Laboratory  of  Houston  &  Kennelly, 
Philadelphia. 


[Copyright,  1894,  by  THE  ELECTRICAL  ENGINEER.] 
WEEKLY. 


No.  20.  OCTOBER  2Y,  1894. 

Electrical   Engineering   Leaflets, 


— BY— 

Prof.  E.  J.  Houston,  Ph.  D. 

AND 

A.  E.  Kennelly,  F.  R.  A.  S. 


INTERMEDIATE  GRADE. 

The  Regulation  of  the  Dynamo. 


163.  In  the  practical  operation  of  dynamos,  whether 
their  circuits  are  intended  for  constant  current  or 
constant  potential  (see  sections  62  to  72)  a  necessity  ex- 
ists for  maintaining  such  constancy  when  the  number  of 
electro-receptive  devices  placed  in  such  circuits  is  varied. 
Such  regulation  of  dynamos  is  accomplished  either  auto- 
matically or  by  hand. 

We  have  seen  that  the  E.  M.  F.  of  a  generator  depends 
on  its  speed,  on  the  number  of  conductors  on  its  arma- 
ture, counted  once  around,  and  on  the  flux  passing 
through  the  armature.  The  speed  and  the  number  of 
conductors  remaining  the  same,  the  variation  of  E.  M.  F. 
is  obtained  either  by  altering  the  flux  through  the  arma- 
ture, by  means  of  a  change  in  the  M.  M.  F.  in  the  coils  of 
the  field  magnets,  or  by  altering  the  position  of  the 
brushes  on  the  commutator,  so  as  to  deliver  into  the 
external  circuit  a  greater  or  lesser  proportion  of  the 
E.  M.  F.  generated  in  the  armature. 

Published  by 

THE   ELECTRICAL  ENGINEER, 
^Broadway,  New  York,  N.  Y. 

([Entered  as  second-class  matter  at  the  New  York,  N.  Y.,  Post  Office,  June  14,  1894.] 


154 


164.  In  constant-current  machines,  which  are  almost 
exclusively  employed  for  series-arc  circuits,  the 
excitation,  that  is,  the  M.  M.  r.  in  the  magnetic  circuit,  is 
usually  maintained  constant,  and  the  variation  of  E.  M.  F. 
is  obtained  by  changing  the  diameter  of  commutation, 
through  the  shifting  of  the  brushes.  Constant-current 
machines  may  be  automatically  controlled  to  supply  any 
number  of  lights,  say,  between  one  and  60,  representing 
a  corresponding  variation  of  E.  M.  F.  of  from  50  to 
3,000  volts. 

In  constant-potential  generators,  which  are  almost  ex- 
clusively employed  for  continuous-incandescent  and 

-f-CUPPLY  MAIN 


FIG.  69. 

Series-wound  generator. 

power  circuits,  the  brushes  are  either  not  moved  at  all, 
or  are  only  slightly  shifted  to  maintain  sparkless  com- 
mutation, and  the  necessary  variation  of  E.  M.  F.  required, 
is  either  obtained  by  altering  the  current  strength  pass- 
ing through  the  field  magnets,  by  means  of  hand  regu- 
lation, or  by  providing  an  additional  field- winding  in  the 
circuit  of  the  armature  ;  that  is,  by  the  compound-wind- 
ing of  the  machine. 

165.     Generators  may  have  the  circuits  of  the  arma- 
ture and  field  magnets  connected  either  in  series 
or  in  parallel.     In  the  former  case,  represented  in  Fig. 


155 


69,  the  generator  is  series-wound.  In  the  latter  case, 
shown  in  Fig.  70,  the  generator  is  shunt-wound.  Here 
a  rheostat  r,  is  inserted  to  control  the  M.  M.  F.  of  the  field- 
magnet.  A  series-wound  machine  tends  to  increase  its 

o 

K.  M.  F.,  as  its  load  increases,  since  then  the  M.  M.  F.  of 
its  magnets  increases,  thus  forcing  more  flux  through 
the  armature  and  raising  its  E.  M.  F.  Shunt- wound  ma- 
chines, on  the  contrary,  tend  to  diminish  their  E.  M.  F. 
as  their  load  increases,  partly  owing  to  drop  of  pressure 
due  to  /7?,  in  the  resistance  of  the  armature,  and  partly 


4- SUPPLY  MA 


4-  SUPPLY  MAIN 


1  SUPPLY  MAI  N  £lec.Evgineer. 

FIG.  70. 

Shunt-wound  generator. 


-  SUPPLY  MAIN 


Elec.  Engineer 


FIG.  71. 

Compound- wound  generator. 


owing  to  the  reduction  of  flux  through  their  armatures, 
as  a  consequence  of  the  reduced  M.  M.  F.  so  effected. 

By  suitably  winding  a  machine,  in  accordance  with 
both  of  the  preceding  types,  the  compound-wound  gene- 
rator, represented  diagrammatically  in  Fig.  71,  can  be 
produced,  which  will  maintain  its  pressure  at  the  ter- 
minals practically  constant  from  no  load  to  full  load. 
The  fall  in  pressure  on  an  increase  of  load,  which  would 
take  place  in  it  considered  as  a  shunt-wound  machine, 
being  offset  by  the  rise  in  pressure,  which  takes  place  in 
it  considered  as  a  series-wound  machine. 


156 

166.     The  following  is  a  classification  of   the  three 
principal  types  of  continuous-current  generators, 
together  with  an  enumeration  of  the  purposes  for  which 
they  are  generally  employed  : 

i  Series-arc  lighting. 

Series- wound •<  Series-incandescent   lighting  (con- 

(      tinuous). 


Generators  •< 


{Central  station  incandescent-paral- 
lel systems. 
Some  motor  circuits. 

f  Isolated  incandescent-parallel  sys- 

Compound-wound.,     gt^ailroad  systems. 
[  Motor  systems. 

167.  The  adjustment  required  in  order  to  maintain, 
automatically,  a  constant  potential  at  the  terminals 

of  a  generator  is  effected  by  ascertaining  the  amount  of 
drop  in  pressure,  which  would  be  produced  if  the  M.  M.  r. 
of  the  machine  were  maintained  constant,  and  then  de- 
termining how  great  an  increase  of  M.  M.  r.  is  necessary 
in  order  to  restore  the  drop  of  E.  M.  r.  This  can  be 
done  by  determining  the  reluctance  of  the  various 
branches  of  the  magnetic  circuit  in  the  dynamo,  and  by 
the  corresponding  Ohm's  law  for  magnetic  circuits, 

cc 

0  =  — ,  calculating   the   increase   in    £F,   necessary   to 

make  the  required  addition  in  <#,  and  also  owing  to  the 
necessarily  increased  reluctance  (R. 

168.  A  very  close  regulation  in  pressure  is  required 
for  the  efficient  operation  of  incandescent  lamps. 

For  example,  if  a  system  of  incandescent  lamps  in  a 
building,  wired  for  115  volts  and  0.43 74  ampere,  each 
(50  watts),  be  steadily  operated  at  116  volts,  i.e.,  at  one 


157 


yolt  above  pressure,  the  illuminating  power  of  the  lamps 
will  be  increased  at  the  outset  to  about  17  candles,  and 
the  average  lifetime  will  be  diminished  about  17  per 
cent.;  while  if  the  pressure  be  permanently  raised  two 
volts,  or  to  117  volts,  the  initial  illuminating  power  will 
be  raised  to  nearly  18  candles,  and  the  lifetime  probably 
reduced  33  per  cent. 

169.  When  several  shunt-wound  generators  are  con- 
nected in  parallel,  as  in  a  central  station  for  sup- 
plying incandescent  lighting,  it  is  essential  to  maintain 
the  E.  M.  F.  of  the  generators  within  close  limits  for 
another  reason.  If  two  shunt- wound  generators  be  run- 
ning in  parallel  with  a  terminal  pressure  of  120  volts, 
with  a  drop  in  the  armature  of,  say,  2.5  volts,  then  a 
diminution  of  speed  amounting  to  two  per  cent,  in  the 
engine  driving  one  of  them,  will  reduce  the  E.  M»  F.  of 
that  machine  to  120  volts.  Under  these  circumstances 
no  current  will  flow  through  the  retarded  generator,  and 
the  lighting  load  will  be  entirely  thrown  off  its  engine. 
The  tendency,  therefore,  will  be  for  the  engine  to  accele- 
rate and  recover  its  share  of  the  load ;  but,  should  this 
not  be  the  case,  and  should  the  engine  continue  to  slacken 
in  speed,  say,  one  per  cent,  further,  the  E.  M.  F.  in  its 
generator  will  fall  below  the  pressure  at  the  terminals  of 
its  neighbor,  and  a  current  will  pass  through  its  arma- 
ture in  the  reverse  direction.  The  result  will  be  that 
the  generator  becomes  a  motor,  and  power  will  be  ex- 
erted towards  driving  its  engine  faster  at  the  expense  of 
load  on  the  remaining  generator.  It  is  evident,  there- 
fore, that  three  per  cent,  of  variation  in  speed,  if  un- 
checked, would  be  sufficient,  under  these  circumstances, 
to  convert  an  active  generator  into  a  motor. 


158 


170.  Fig.  72  shows,  diagrammatically,  the  connections 
for  two  shunt-wound  generators  arranged  for 
operation  in  parallel.  In  starting,  one  machine  only, 
say  A,  is  operated,  its  own  engine  being  brought  up  to 
speed,  and  the  resistance  y,  entirely  cut  out,  leaving  the 
shunt  winding  directly  connected  to  the  brushes.  This 
enables  sufficient  current  to  be  produced  in  the  armature, 
under  the  influence  of  the  residual  magnetic  flux  in 
the  circuit,  to  generate  increasing  M.  M.  F.  in  the  mag- 
netic circuit,  and  from  this,  an  increasing  E.  M.  F.  of 
the  armature. 


FIG.  72. 

Connections  of  two   hand  regulated 
shunt-wound  generators  in  parallel. 


FIG.  73. 

Connections  of  two  compound  wound, 
self-regulating,  generators  in  parallel. 


This  mutual  action  and  reaction,  between  the  electric 
and  magnetic  circuits,  produces  an  accumulated  E.  M.  F., 
or,  as  it  is  commonly  called,  a  "  building-up  "  of  E.  M.  F. 
As  soon  as  the  machine  attains  its  full  pressure,  as  indi- 
cated by  a  voltmeter  connected  with  the  brushes,  suffi- 
cient resistance  in  the  rheostat  /*,  is  introduced  into  the 
circuit  to  maintain  that  pressure,  and  then  the  switch  «, 
is  closed,  thus  connecting  the  E.  M.  F.  of  the  machine 
with  the  bus  bars  +  and  — .  As  soon  as  the  load  be- 
comes too  great  for  a  single  generator,  the  second  ma- 
chine B,  is  thrown  into  action,  by  running  its  engine  up 
to  speed,  short-circuiting  its  rheostat,  building-up  its 


159 


E.  M.  F.,  adjusting  that  E.  M.  F.  by  the  rheostat  to  the 
pressure  on  the  mains,  and  then,  at  that  pressure,  closing 
the  switch  s1.  A  slight  increase  in  the  E.  M.  F.  of  the 
machine  B,  will  enable  it  to  then  share  the  load  with  A, 
and  the  final  adjustment  is  usually  made  by  the  observa- 
tion of  ammeters  in  their  respective  circuits.  When  the 
load  diminishes  sufficiently  to  permit  a  single  generator, 
say  A,  to  sustain  it,  the  reverse  steps  are  followed ; 
namely  B,  has  its  pressure  lowered  by  means  of  the 
rheostat  rly  until  little  or  no  current  passes  from  this 
machine  to  the  bus  bars.  The  switch  s\  is  then  opened 
and  the  engine  driving  B,  is  then  stopped.  The  brushes 
of  the  slackening  machine  are  never  lifted,  nor  the  field 
circuit  broken,  until  the  machine  is  at  rest,  lest  the 
powerful  E.  M.  F.  of  self-induction,  set  up  by  suddenly 
breaking  the  field  circuit,  should  damage  the  insulation 
of  field  or  armature. 

171.  Fig.  73  shows,  diagrammatically,  the  action  of 
two  compound-wound,  self-regulating  generators, 

arranged  for  connection  to  the  bus  bars  in  parallel. 
Here  the  same  steps  are  followed  as  before  for  connect- 
ing the  machines  successively  to  the  circuit,  except  that 
little  or  no  adjustment  is  necessary  in  the  shunt  cir- 
cuit, the  pressure  being  automatically  maintained  by  the 
coarse  and  fine  windings  of  the  field  ;  the  machine  will 
tend  to  divide  the  load  if  the  engines  are  well  governed 
and  uniformly  driven. 

172.  The    danger   of    employing   shunt- wound    ma- 
chines for  incandescent  lighting,  is  that,  if  a  short- 
circuit  takes  place  between  the  mains,  the  tendency  will 
be  for  the  field  magnets  to   become  thereby  weakened, 


100 


owing  to  the  heavy  drop  at  the  machine  terminals  and  the 
reduction  of  M.  M.  F.  by  armature  reaction.  This  may 
often  act  as  a  safeguard  to  the  armature  against  a  dan- 
gerously strong  current.  If,  however,  the  machine  be 
compound-wound,  as  in  Fig.  71,  a  short-circuit  will  not 
tend  to  demagnetize  the  field  magnets,  and  the  current 
will  increase  until  either  the  fuse  in  the  circuit  is  melted, 
until  the  engines  are  stopped,  or  until  a  breakdown 
occurs  in  the  circuit. 

SYLLABUS. 

In  practice,  generators  are  usually  required  either  to 
maintain  a  constant  current  under  all  variations  of  E.  M.  F., 
or  a  constant  E.  M.  F.  under  all  variations  of  current. 

The  automatic  regulation  of  pressure,  in  a  constant- 
current  machine,  is  almost  invariably  effected  by  auto- 
matically shifting  the  position  of  its  brushes  on  the 
commutator. 

The  automatic  regulation  of  pressure  in  a  constant- 
potential  machine  is  almost  invariably  effected  by  com- 
pound-winding. 

In  constant-potential  machines,  supplying  incandescent 
lamps,  close  regulation  is  desirable  in  order  to  operate 
the  lamps  to  their  best  advantage. 

Laboratory  of  Houston  &  Kennelly, 
Philadelphia. 


(.Copyright,  1894,  by  THE  ELECTRICAL  ENGINEER.] 
WEEKLY. 

No.  21.  NOVEMBER  3,  1894. 

Electrical    Engineering   Leaflets, 


Prof.  E.  J.  Houston,  Ph.  D. 

AND 

A.  E.  Kennelly,  F.  R.  A.  S. 


INTTE^JYIEDIATTE   CRAJDE. 

ELECTRODYNAMICS. 


173.  Electrodynamics  is  that  branch  of  electricity 
which  treats  of  the  mutual  attractions  between 
neighboring  electrical  conductors  traversed  by  currents, 
or  between  such  electrical  conductors  and  magnets. 
Magnetodynamics,  sometimes  included  under  the  head 
of  electrodynamics,  treats  of  similar  attractions  and  re- 
pulsions between  neighboring  magnets.  The  phenomena 
of  electrodynamics  and  of  magnetodyiiamics  are  essenti- 
ally the  same. 

For  example,  in  a  continuous  current  electromagnetic 
motor,  the  turns  of  conductor  carrying  currents  and 
supported  upon  the  armature,  are  apparently  attracted  by 
the  poles  of  the  h'eld  magnets,  and,  in  obedience  to  such 
forces,  the  armature  is  set  in  rotation.  So  too,  when  a 
small  compass  needle  is  placed  near  a  conductor  carrying 
a  current,  the  needle  tends  to  set  itself  at  right  angles 
to  the  conductor  in  obedience  to  the  laws  of  electrodyna- 

Published  by 

THE    ELECTRICAL   ENGINEER, 
203  Broadway,  New  York,  N.  Y. 

[Entered  as  second-class  matter  at  the  New  York,  N.  Y.,  Post  Office,  June  14,  1894.} 


162 


mics,  and  the  force  by  which  it  is  turned  on  its  pivot  is 
the  electrodyn  am ic  force. 

Fig.  74:  shows  a  conductor  of  length  7,  cms.  situated 
in  a  uniform  magnetic  flux  of  intensity  (B,  gausses,  at 
right  angles  to  the  conductor.  If  now  a  current  of 
strength  ^',  amperes  be  sent  through  the  conductor,  then 
the  conductor  will  be  acted  upon  by  an  electrodynamic 
force 

dvnes. 


10 


The  direction  in  which  this  force  is  exerted  is  represented 


FIG.  75. 

Diagram  indicating  the  direction  of  the 
electromagnetic  force  on  an  active  con- 
ductor lying  across  a  uniform  magnetic 
flux. 

by  the  arrow  o  c,  and,  if  the  direction  of  the  current 
be  reversed,  or  if  the  direction  of  the  flux  be  reversed, 
the  direction  of  the  force  will  be  reversed. 

The  electrodynamic  force  depends  : 

(1.)  Upon  the  length  of  the  wire  at  right  angles  to  the 
flux. 

Upon  the  intensity  of  the  magnetic  flux. 
Upon  the  strength  of  the  current  passing  through 
the  conductor. 

No  work  will   be  done  by  the  electrodynamic  force 


unless  the  conductor  moves  in  obedience  to  it.  If  the 
conductor  is  prevented  from  moving,  the  force  will  do 
no  work. 

174.  The  direction  of  the  mechanical  force  exerted 
upon  the  active  conductor  may  be  determined  by 

Fleming's  hand  rule  for  motors,  which  differs  from  his 
hand  rule  for  generators  in  that  the  left  hand  is  employed. 
Here,  if  the/breflnger  indicates  the  direction  of  the/lux, 
the  middle  linger  the  direction  of  the  current  in  the 
conductor,  then  the  thumb  will  indicate  the  direction  of 
the  motion  produced  by  the  electromagnetic  force,  as 
in  Fig.  75. 

175.  In  treating  of  induced  E.  M.  F.,  (Sec.  132),  it  was 
observed  that  no  current  could  be  induced  in  a 

conductor  cutting  through  a  magnetic  flux  without  hav- 
ing a  complete  loop  or  circuit,  and  that  the  law  of  E,  M.  F. 
for  a  short  length  of  the  wire,  resolved  itself  into  the 
more  general  law  dealing  with  the  entire  loop.  So  too 
in  the  case  of  electrodynamic  force,  the  law  of  the  force 
exerted  upon  any  short  length  of  active  conductor  re- 
solves into  a  more  general  law  dealing  with  the  entire  loop 
which  any  such  active  conductor  must  necessarily  form. 

176.  We  have  seen  that  whenever  "forces  are  applied 
electrodynamically  to  a  loop  or  circuit,  they  do  no 

work  until  motion  is  produced.  (Sec.  13.)  The  amount 
of  work  done  in  any  motion,  expressed  in  ergs,  will  be  the 
product  of  the  current  strength  in  amperes,  by  the  increase 
in  flux  (webers)  linked  with  the  current,  divided  by  10,  or, 

ergs,  or,  ^£555  joules, 

since  10  megergs  make  one  joule. 


W  = 


164 


FoF  example,  Fig.  76  represents  a  horizontal  loop  of 
conductor  AB  c  D,  carrying  a  current  of,  say,  20  amperes, 
and  situated  upon  the  surface  of  a  motor  armature,  in  a 
uniform  external  flux  supplied  by  bipolar  lield-magnets. 
Under  the  electromagnetic  force  acting  upon  this  loop  it 
rotates  upon  its  axis  until  it  occupies  a  vertical  position 
abed,  in  which  it  contains  say  10  megawebers  of  flux. 
Then  the  work  done  by  the  armature  during  this  quarter 
revolution,  owing  to  the  electrodynamic  action  of  this  turn, 
will  be  20  X  10,000,000  =  20)000)000  ergg  =  2  joules  = 

1.476  Ibs.  lifted  one  foot  at  Washington.     This  amount 


FIG.  76. 

Diagram  of  rectangle  of  active  conductor  A  B  c  D,  situated  in  a  uniform  magnetic  flux. 

of  energy  has  been  absorbed  from  the  source  of  electric 
current  supplied  to  the  armature,  and  has  been  developed 
against  the  E.  M.  F.  established  in  the  turn  of  wire  by  its 
motion  through  the  magnetic  flux.  If  the  circuit  instead 
of  forming  a  single  loop,  consists  of  a  number  of  loops, 
as,  for  example,  a  coil,  then  the  amount  of  work  done  by 
the  electromagnetic  forces  upon  such  an  assemblage  of 

loops  is  the  sum  of  - —  for  each  loop  separately,  and,  if 
the  increase  of  flux  be  equal  for  all  the  loops,  the  total 


165 


-   () 
amount  of  work  will  be  - — ,  multiplied  by  the  number 

of  loops.  If  there  were  600  loops  of  wire  on  the  arma- 
ture of  the  motor  previously  considered,  each  turn  would 
exert  two  joules  of  work  in  a  quarter  revolution,  and  the 
total  work  expended  would  be  1,200  joules  per  quarter 
revolution,  or  4,800  joules  per  revolution.  If  the  motor 
made  720  revolutions  per  minute,  or  12  revolutions  per 
second,  it  would  develop  4800  X  12  =  57,600  joules  per 
second,  or  57.6  KW. 

The  work  done  by  an  electrodynamic  force  is  invari- 
ably obtained  from  the  circuit  or  circuits  in  which  the 
force  is  produced.  Thus,  when  a  loop  moves  in  a  mag- 
netic flux,  its  motion  induces  an  E.  M.  F.  in  the  loop, 
which  is  opposed  to  the  direction  of  the  current  in  the 
loop,  and  if  this  E.  M.  F;  be  denoted  by  <?,  volts  the  work 
is  absorbed  by  the  loop  from  the  source  of  current  at  the 
rate  of  e  i,  watts. 

177.  The  tendency  of  electrodynamic  forces  is  to 
bring  the  external  or  prime  flux  into  parallelism 
with  the  flux  produced  by  the  current  in  the  active  loop. 
The  loop  endeavors  to  embrace  as  much  flux  as  it  can  in 
the  same  direction  as  that  produced  by  its  own  M.  M.  F. 
If  the  external  or  prime  flux  passes  through  the  loop  in 
the  opposite  direction  to  that  produced  by  its  own  M.  M.  F., 
the  loop  will  tend  to  diminish  or  expel  the  external  flux, 

and  the  force  so  exerted  will  be  -      ergs,  as  before,    0, 

being  now  reckoned  as  flux  expelled  and  not  as  flux 
linked.  Thus,  in  a  motor  armature  when  a  given  loop 
reaches  the  vertical  position,  it  will  contain  a  maximum 
amount  of  flux  from  the  field  magnet  circuit,  and,  no 


166 


increase  in  the  current  carried  by  the  loop,  will  produce  a 
further  rotary  force ;  but,  if  by  the  action  of  the  commu- 
tator, the  current  in  the  loop  is  reversed  at  the  moment 
when  it  reaches  the  vertical  position,  the  flux  will  now 
pass  through  the  loop  in  the  reverse  direction  to  that  pro- 
duced by  its  own  M.  M.  F.,  and  a  new  electrodynamic  force 
is  exerted  upon  the  loop  until  it  is  again  filled  with  flux 
in  the  direction  parallel  to  that  from  its  own  M.  M..  F. 

ITS.  When  the  loop  on  the  motor  armature  has 
reached  a  vertical  position  in  which  it  contains  a 
maximum  flux,  and  therefore  exerts  no  force,  it  would 
not  be  carried  past  this  point,  which  would  constitute  a 
dead  point  were  it  not  for  the  momentum  of  the 
armature.  By  winding  a  large  number  of  turns  on  the 
armature  at  equal  angular  distances  apart,  the  torque  of 
the  armature,  i.e.,  its  rotary  effort  as  measured  by  the 
tangential  pull  referred  to  unit  radius,  is  rendered  uni- 
form, and  momentum  is  no  longer  depended  upon  for 
its  passage  past  its  dead  point. 

179.     We  have  seen  that  the  work  done  by  electro- 
dynamic   forces   on  a  loop,  is  expressed  by  — - 

dynes,  where  <#,  is  the  flux  admission  expressed  in  webers. 
If,  therefore,  the  mechanical  system  of  the  loop  is  such, 
that  the  only  possible  motion  is  a  continued  rotary  mo- 
tion about  the  axis,  as  in  a  motor  armature,  then  for  any 
given  small  angular  rotation,  the  work  done  will  obviously 
be  great  when  the  flux  admission  in  that  small  rotation 
is  great,  and  will  be  small  when  the  flux  admission  is 
small ;  that  is  to  say,  the  force  exerted  through  that 
angular  displacement,  will  depend  upon  the  rate  of  flux 
admission.  The  torque,  or  tangential  force  at  unit  radius, 


16T 


exerted  by  the  loop  about  the  axis  of  rotation,  is  equal 
to  the  current  strength  in  amperes,  divided  by  20  TT,  and 
multiplied  by  the  flux  admission  per  unit  angular  dis- 
placement. 

180.  The  electrodynamic  forces  here  described  are 
not  confined  to  the  mutual  interactions  of  inde- 
pendent fluxes,  but  are  produced  by  the  action  of  a  single 
electric  circuit.  If  a  current  be  sent  through  the  loop 
of  wire  as  shown  at  A  in  Fig.  YT,  the  loop  tends  to  spread 
outwards,  as  shown  by  the  dotted  arrows,  so  as  to  em- 


B    d 


Diagrams  indicating  the  direction  of  electromagnetic  forces  in  a  loop,  and  between 
parallel  wires  due  to  the  flux  linked  with  the  circuit. 

brace  more  of  its  own  flux,  and,  if  the  circuit  be  com- 
posed of  several  loops,  these  will  tend  to  move  together, 
and  always  extend  outwards  so  as  to  embrace  as  much  of 
each  other's  flux,  and  also  as  much  of  their  own  flux  as 
possible.  In  other  words,  the  entire  circuit  tends  to 
move  so  as  to  contain  as  much  flux  as  possible.  If, 
therefore,  a  free  spiral  or  helix  be  traversed  by  a  current 
it  will  tend  to  shorten  or  contract. 

Similarly,  two  parallel  active  conductors,  as  at  c,  Fig, 


Of  THS 


168 


77,  carrying  currents  in  opposite  directions,  are  urged 
apart  by  electrodynamic  force,  while  if,  as  at  B,  the  cur- 
rents flow  in  the  same  direction,  the  wires  are  urged  to- 
gether. In  the  former  case  the  loop  formed  by  two 
wires  a  1)  and  c  d,  widens  so  as  to  embrace  more  flux. 
In  the  latter  case  the  loops  formed  by  the  circuits  of  a'  b' 
and  cf  d',  have  the  flux  they  embrace  mutually  increased 
by  the  approach  of  the  two  wires. 

SYLLABUS. 

Electrodynamic  force  exerted  upon  an  active  conductor 
situated  in  a  magnetic  flux,  depends  on  the  length  of  the 
conductor  exposed  across  the  flux,  on  the  intensity  of 
the  flux,  and  on  the  strength  of  the  current  in  the  con- 
ductor. 

The  work  done  by  electrodynamic  forces  in  the  mo- 
tion of  active  conductors  is  supplied  by  the  electric 
source  whose  E.  M.  F.  sends  the  current  through  the  con- 
ductor, and  is  expended  by  the  source  in  work  done  by 
the  current  against  the  E.  M.  F.  induced  in  the  conductor 
during  its  motion  through  the  external  flux. 

The  work   done  by  an  active  loop  in  any  motion  in 

consequence    of  electrodvnamic    forces   is 

100,000,000 

joules,  where  *',  is  the  strength  of  the  current  in  amperes, 
and  <0,  is  the  flux  admission  in  webers.  If  <#,  be  opposed 
in  direction  to  the  flux  from  the  M.  M.  F.  of  the  wire, 
flux  expulsion  is  equivalent  to  admission  in  its  own  direc- 
tion. 

An  electric  motor  is  a  machine  for  the  development 
of  uniform  rotary  motion  by  electrodynamic  forces. 

Laboratory  of  Houston  &  Kennelly, 
~  Philadelphia, 


[Copyright,  1894,  by  THK  ELECTRICAL  ENGINEER.] 
WEEKLY. 


No.  22.  NOYEMBEK  10,  1894. 

Electrical   Engineering   Leaflets, 


— BY— 

Prof.  E.  J.  Houston,  Ph.  D. 

AND 

A.  E.  Kennelly,  F.  R.  A.  S. 


INTERMEDIATE  GRADE. 

THE  ELECTRIC  MOTOR 

(CONTINUOUS    CURRENT   TYPE,) 


181.  A  dynamo-electric  machine  may  be  operated 
either  as  a  dynamo  or  as  a  motor  ;  that  is  to  say, 
any  generator  can  be  operated  as  a  motor,  and  any  motor 
operated  as  a  generator,  provided  proper  arrangements 
are  secured  for  the  excitation  of  the  field  magnets.  The 
reason  for  this  reversibility  of  the  dynamo  and  motor  is 
that  electromotive  and  electrodynamic  fcrces  coexist  in 
each.  In  the  dynamo,  the  E.  M.  F.  is  produced  by  the 
rotation  of  the  loops  on  its  armature  through  the  flux 
from  the  field  magnets,  but  so  soon  as  that  E.  M.  F.  is  per- 
mitted to  send  a  current  through  an  external  circuit, 
electrodynamic  forces  are  set  up  by  the  current  in  the 
armature  under  the  mutual  interaction  of  the  field  and 
armature  fluxes,  and  power  has  to  be  applied  to  the 
dynamo  to  drive  it.  The  pressure  at  the  terminals  of  the 
machine  will  be  less  than  the  E.  M.  F.  within  the  machine, 
by  an  amount  equal  to  the  drop  in  the  machine,  or  i  r, 
where  r  is  the  resistance  of  the  machine,  and  the  electro- 


Published  by 

THE   ELECTRICAL  ENGINEER, 
203  Broadway,  New  York,  N.  Y. 

|_Entered  as  second-class  matter  at  the  New  York,  N.  Y.,  Post  Office,  June  14,  1894.} 


iro 


dynamic  force  is  opposed  to  the  rotation  of  the  machine, 
and  may  be  called  the  counter-dynamic  force. 

182.  On  the  other  hand,  when  a  current  is  applied  to 
a  motor,   the   current   causes   the   loops  on   the 

armature  to  revolve  through  the  flux  from  the  field 
magnets.  The  loops  being  filled  with  and  emptied  of  flux, 
during  this  revolution,  generate  an  E.  M.  F.  opposed  to 
the  direction  of  the  current  and,  called  a  counter  E.  M.  F. , 
usually  abbreviated  c.  E.  M.  r.  The  pressure  at  the  ter- 
minals of  the  motor  E,  volts,  will  be  greater  than  the 
c.  E.  M.  F.,  e  volts,  by  the  amount  equal  to  the  drop  in  the 
machine  or  i  r,  volts. 

The  distinctive  feature  of  difference  between  the  gen- 
erator and  the  motor,  is  that  in  the  generator  the  current 
is  in  the  direction  of  the  E.  M.  F.,  while,  in  the  motor  it 
is  opposed  to  the  E.  M.  F.  of  the  machine.  In  one  case 
the  output  of  the  machine  is  E  i  watts,  and  in  the  other 
case  the  intake  is  E  i  watts. 

183.  When  a  motor  is  running,  as  in  the  case  of  a 
generator,    (see  Sec.  144,)  its  c.  E.  M.  F.  is   the 

product  of  the  number  of  revolutions  per  second,  the 
number  of  wires  on  the  surface  of  the  armature,  counted 
once  around,  and  the  total  flux  in  webers  passing 
through  each  pole  into  the  armature,  and  divided  by 
100,000,000.  Thus,  if  the  25  KW.  bipolar  motor  repre- 
sented in  Fig.  78,  makes  900  revolutions  per  minute,  or  15 
revolutions  per  second,  and  if  there  are  200  wires  lying 
on  the  surface  of  the  armature,  counted  once  around, 
while  four  mega  webers  pass  tli  rough  each  pole  into,  or 
out  of,  the  armature,  the  c.  K.  M.  F.  of  the  motor  will  be, 
4,000,000  X  200  X  15  = 
100,000,000 


171 


If  the  drop  in  the  armature  of  this  machine  due  to 
i  r,  be  five  volts,  the  pressure  at  its  brushes  will  be  125 
volts. 

184.  The  controlling  factors  in  the  operation  of  a 
motor  are  its  speed,  and  its  torque,  and  these  vary 
greatly  in  different  cases  according  to  the  amount  and 
character  of  the  work  that  has  to  be  performed.  These 
conditions  may  be  arranged  under  the  following  general 
classes;  namely, 


FIG.  78. 

(1.)  Cases  in  which  a  constant  torque  and  constant 
speed  are  required. 

(2.)  Cases  in  which  a  constant  torque  and  variable 
speed  are  required. 

(3.)  Cases  in  which  a  variable  torque  and  a  constant 
speed  are  required. 

(4.)  Cases  in  which  a  variable  torque  and  variable 
speed  are  required. 

Instances  of  the  first  case  are  seen  in  fan  motors  and 
in  rotary  pumps. 


172 


Instances  of  the  second  case  are  seen  in  hoisting  ma- 
chinery, elevators  and  rollers. 

Instances  of  the  third  case  are  seen  in -most  machines 
and  tools. 

Instances  of  the  fourth  case  are  street  car  motors. 

185.     If  the  pulley  p,  Fig.  79,  be  keyed  to  the  arma- 
ture shaft  of  a  motor    which  is  turning  in  the 
direction  indicated  by  the  arrow,  it  will  raise  the  weight 
w,  and  do  work.     The  torque  exerted  by  the  pulley  will 
be  the  weight  multiplied  by  the  effective  radius  of  the 


I "  •  i 
Fm.  79.  PIG.  80. 

Motor  pulley  lifting  a  weight.  Motor  pulley  driving  a  pulley  P,  by  means  of  a  belt. 

pulley.  For  example,  if  the  pulley  have  a  diameter  of 
11  inches,  its  radius  will  be  5.5"  and  if  the  rope  have  a 
diameter  of  1",  the  effective  radius  of  the  pulley  will  be 
increased  by  half  the  thickness  of  the  rope  ;  or  to  6"  =  0.5 
foot.  If  the  weight  w,  be  500  pounds,  the  torque  at 
the  motor  shaft  will  be  0.5  X  500  =  250  pounds-feet. 
Or,  in  other  words,  is  equal  to  the  weight  of  250  pounds 
supported  at  the  effective  unit  radius  of  one  foot.  If  the 
motor  be  employed  to  lift  this  weight,  the  rate  at  which 


173 


it  will  lift  it  will  be  the  number  of  revolutions  per  sec- 
ond multiplied  by  the  effective  circumference  of  the 
pulley.  In  this  case,  for  every  revolution  of  the  pulley, 
the  rope  will  be  lifted  through  the  distance  of  2  /r  X 
0.5  =  3.1416  feet ;  and,  if  the  armature  makes  20  revo- 
lutions per  second,  the  rate  of  lifting  will  be  62.832 
feet  per  second.  The  work  done  per  second,  that  is,  the 
activity  of  raising,  will  be  62.832  X  500  =  31,416  foot- 
pounds per  second ;  and,  since  550  foot-pounds  per 
second  represents  the  activity  of  one  horse-power,  and 
737.3  foot-pounds  per  second  represents  the  activity  of 

one  kilowatt,  the  output  of  the  motor  will  be      '         = 

7o7.o 

42.61  KW. 

186.  If,  as  shown  in  Fig.  80,  the  pulley  M,  keyed  to 
the  motor  shaft,  drives  a  countershaft  pulley  F, 
by  means  of  the  belt  s  T,  which  moves  in  the  direction 
of  the  arrows,  there  will  be  two  forces  acting  on  each 
pulley  instead  of  a  single  force,  as  represented  in  the 
preceding  figure  ;  namely,  the  forces  on  the  two  parts 
of  the  belt.  One  of  these,  the  lower,  marked  T,  is,  how- 
ever the  tight  or  driving  side,  while  the  upper,  or  s,  is 
the  slack  or  following  side,  and  the  difference  between 
the  two  tensions  exerted  by  the  belt  represents  the  cor- 
responding equivalent  pull  of  the  preceding  case.  Thus, 
if  the  tension  on  the  side  T,  be  equal  to  1,000  pounds 
weight,  while  the  tension  on  the  side  s,  be  equal  to  400 
pounds  weight,  the  effective  pull  will  be  600  pounds 
weight,  delivered  at  the  periphery  or  effective  radius  of 
the  pulley  M,  while  the  sum  of  the  tensions  or  1,400 
pounds,  will  be  exerted  in  drawing  the  shafts  bodily  over 
against  their  journal  bearings. 


174 


The  torque  exerted  by  the  motor  armature  is 


10  X  2* 

cm.-dynes,  where  i,  is  the  current  strength  through  the 
armature  of  the  motor  in  amperes ;  <#,  the  flux  passing 
through  each  pole  into,  or  out  of,  the  armature  in  webers ; 
and  w9  the  number  of  wires  lying  on  the  surface  of  the 
armature,  counted  once  around.  Thus,  in  the  case  of  the 
motor  already  considered,  if  the  current  through  the 
armature  were  50  amperes,  the  torque  exerted  by  the 
motor  would  be, 
50  X  4,000  OOP  X  200  =  636)700)000  ^^  nearly. 

62.832 

so  that,  if  the  effective  radius  of  the  pulley  were  1  cm., 
the  motor  would  just  exert  a  force  at  the  periphery  of 
the  pulley  of  636,700,000  dynes ;  and,  since  a  dyne  is 
a  force  equal  to  1.0203  milligrammes  weight,  the  motor 
would  just  lift  649,600,000  milligrammes  =  649.6  kilo- 
grammes =  1,432  pounds  weight  at  an  effective  radius 
of  one  cm.,  since  one  kilogramme  =  2.205  pounds.  For 
a  pulley  whose  effective  radius  was  one  foot,  (30.48  cms.), 

however,  the  motor  would  raise  — ' —  =  46.98  pounds. 

30.48 

In  this  calculation  we  have  to  consider  that  the  rotating 
motor  armature  exerts  a  torque  partly  expended  in 
overcoming  mechanical,  electrical,  and  magnetic  frictions, 
so  that  the  resisting  torques  due  to  these  causes  must 
be  subtracted  in  order  to  arrive  at  the  available  or  useful 
mechanical  torque. 

Thus,  if  the  torque  exerted  by  this  motor  against  me- 
chanical friction  were  3  pounds- feet,  against  hysteresis  2 
pounds-feet,  and  against  eddy-current  electrodynamic 
forces  1  pound-foot,  the  mechanical  torque  at  the  pulley 


1-75 


when  running  with  50  amperes  through  the  armature 
would  be  40.98  pounds-feet. 

187.     In  Fig.  81  is  represented  a  case  of  the  transmis- 
sion of  a  power  of  20  KW.  to  a  distance  of  one 
mile,  from  an  engine  to  a  line  shaft,  with  the  aid  of  two 


Generator 

Output  30T20  K.W.=  10l4t?iJ  H.P. 
Intake  373iK.W  =50  &  H.P. 
Efficiency  80  * 


Motor 

Output  20  K.W.)  P~  .         „,.,, 
Intake  25  K.W.iEaclcI1(*80£ 

C.E.M.F.  of  motor  475  voltg 
Drop  in  motor  armature  25  volts 
Bes.  of     "  "        Uohm 

Torque  of    »  «       igb  Ibs.  at  1  feot  ruliui 


Line  Shaft  Pulley 
Power  delivered  26I80H.P.-20  K.W. 


Engine  Delivery  STJiK.W. 
Motor  Delivery  20  K.W. 
Nett  efficiency  nearly  53* 


FIG.  81. 


similar  500  volt  dynamo  machines,  one  employed  as  a 
generator,  and  the  other  as  a  motor,  their  efficiency  being 
taken  as  80  per  cent.,  and  the  net  efficiency  of  the  sys- 
tem 53  per  cent. 


ire 


SYLLABUS. 

Dynamos  and  motors  are  reversible  machines,  in  all 
cases  where  suitable  means  are  provided  for  exciting 
their  field  magnets. 

In  both  dynamos  and  motors  electromotive  forces  and 
electrodynamic  forces  coexist.  in  the  dynamo  the 
E.  M.  F.  is  direct,  or  aids  the  current,  while  the  dynamic 
force  is  opposed  to  the  motion,  or  is  a  counterdynamic 
force.  In  a  motor  the  E.M.  F.  is  opposed  to  the  current, 
or  has  a  c.  E.  M.  F.,  while  the  dynamic  force  is  direct,  or 
exerts  rotation. 

In  the  dynamo,  the  terminal  pressure  is  less  than  the 
E.  M.  F.  in  the  armature.  In  the  motor,  the  terminal 
pressure  is  greater  than  the  c.  E.  M.  F. 

Laboratory  of  Houston  &  Kennelly, 
Philadelphia. 


[Copyright,  1894,  by  THK  ELECTRICAL  ENGnmsit.'] 
WEEKLY. 


No.  23.  NOVEMBER  17,  1894. 

Electrical   Engineering   Leaflets, 


Prof.  E.  J.  Houston,  Ph.  D. 

AND 

A.  E.  Kennelly,  F.  R.  A.  S. 


INTERMEDIATE  CRADE. 

THE  ELECTRIC  MOTOR 

(CONTINUOUS    CURRENT   TYPE.) 


188.  When  the  torque  exerted  by  a  motor,  supplied 
direct  from  constant-potential  mains,  is  constant, 
as,  for  example,  when  the  motor  is  applied  to  lifting  a 
weight  in  an  electric  hoist  (see  Fig.  82),  and  when  at  the 
same  time  the  speed  of  lifting  has  to  be  varied,  there 
are  practically  two  ways  of  obtaining  the  required  varia- 
tion in  speed,  viz., 

(1.)  By  introducing  a  rheostat  into  the  armature  cir- 
cuit, thereby  producing  a  drop  of  pressure  in  that  circuit, 
and  reducing  the  c.  E.  M.  F.  which  the  armature  has 
to  make  up ;  that  is,  reducing  the  speed  at  which  the 
motor  has  to  run. 

(2.)  By  varying  the  M.  M.  F.  of  the  field  magnets,  so 
as  to  produce  a  varying  flux  through  the  armature,  and, 
consequently,  a  varying  c.  E.  M.  F. 

The  first  method  is  capable  of  being  applied  over  any 
desired  range,  but  is  wasteful  of  energy.  The  second 
method  does  not  waste  energy,  but  is  only  capable  of 
being  practically  applied  over  a  limited  range. 


Published  by 

THE  ELECTRICAL  ENGINEER, 
903  Broadway,  New  York,  N.  Y. 

I. Entered  as  second-class  matter  at  the  New  York,  N.  Y.,  Post  Office,  June  14,  1894.] 


178 


189.     We   have   seen   that   the   torque   of    a   motor 


armature  is  equal  to 


w 


20 


cm.-dynes  (Sec.  186), 


including  the  frictional  torque  of  the  armature ;  conse- 
quently, with  a  fixed  armature  flux  0,  a  given  torque 
requires  a  definite  current  strength  ?',  amperes.  The  en- 
ergy supplied  from  the  mains  to  obtain  this  torque,  must, 
therefore,  be  E '  i  watts,  where  E  is  the  pressure  in  the 


FIG.  82. — SMALL  ELECTRIC  MINE  HOIST. 

mains.  Consequently,  this  amount  of  energy  must  be 
taken  from  the  mains,  whether  the  motor  does  work  or 
not ;  that  is  to  say,  whether  it  merely  exerts  the  required 
torque  without  moving,  or,  whether  it  moves  and  does 
work  at  such  a  speed  that  the  c.  E.  M  F.  is  nearly  equal 
to  £,  and  no  external  drop  in  resistance  has  to  be  sup- 
plied. 

The  conditions  of  operating  a  motor  with  external  re- 


179 


sistance  in  its  circuit,  are  necessarily  unstable,  except 
with  a  perfectly  uniform  torque  ;  for,  any  variation  of 
torque  will  cause  the  motor  to  demand  a  varied  current 
strength,  and  the  drop  in  the  resistance  will  corres- 
pondingly vary  with  changes  in  the  current,  necessita- 
ting a  change  in  the  speed  and  c.  E.  M.  F.  of  the  motor 
to  maintain  balance  of  pressure.  Moreover,  in  large 
machines,  which  have  to  supply  a  powerful  torque,  a 
powerful  current  may  be  necessary,  and  the  amount 
of  energy  which  has  to  be  dissipated  in  heating  the 
external  resistance,  when  the  motor  is  operating  at  its 
lowest  speed,  may  be  very  considerable,  requiring  a  cum- 
bersome and  costly  rheostat,  in  order  to  carry  safely  the 
full-load  current,  and  dissipate  the  heat  generated  by  the 
same. 

190.     If  it  were  possible  to  vary  widely  the  range  of 

the  armature  flux   in  a   motor,   constant  torque 

could  be  maintained  at  varying  speeds  without  waste  of 

energy  ;  for,  the  torque  being  equal  to  - ??,  a  large  in- 
crease in  <#,  would  necessitate,  for  the  same  torque,  a 
correspondingly  large  decrease  in  i,  the  current  strength ; 
while,  since  also  $  n  w,  represents  the  c.  E.  M.  F.  of  the 
motor,  the  same  large  increase  in  0,  would  necessitate  a 
correspondingly  large  reduction  in  n,  the  number  of  re- 
volutions of  the  armature  per  second ;  so  that  the  cur- 
rent and  energy  absorbed  would  diminish  along  with  a 
diminution  of  speed.  Thus,  if  a  shunt  motor  be  oper- 
ated from  constant  potential  mains  at  125  volts  pressure, 
and  the  full-load  current  be  100  amperes,  so  that  its  full- 
load  intake  in  the  armature  circuit  is  12.5  KW.,  the  speed 
of  the  motor  would  be  900  revolutions  per  minute,  or  15 


180 


revolutions  per  second,  if  the  resistance  of  the  armature 
Were  0.05  ohm,  the  number  of  wires  on  the  armature 
surface  200,  and  the  total  useful  flux  through  one  pole 
4  megawebers  ;  for,  the  drop  in  the  armature  at  full-load, 
would  be  100  X  0.05  =5  volts,  and  the  c.  E.  M.  F.  120 
volts,  so  that  0  n  w  is 

4,000,000  X  15  X  200  =  120  X  108      the  c.  E.  M.  F. 
The  torque  exerted  by  the  armature,  would  be 

i  <Pw     100  X  4,000,000  X  200 
r= = '         — — &1.273X109     cm.-dvnes. 

20  n  02.83 

This  would  be  the  total  torque  of  the  armature  includ- 
ing the  torque  exerted  against  frictions  in  the  machine. 
If  these  constituted  ten  per  cent,  of  the  total,  or  0.1  ^TH 
X  109  cm.-dynes,  the  available  torque  at  the  pulley 
would  be  1.146  X  109  cm.-dynes  =  84.44  pounds-feet. 
At  full  speed  the  motor  would  do  work  at  the  pulley 
amounting  to  2  n  n  r,  or  900  X  0.283  X  84.44  =  477,<>00 
foot-pounds  per  minute  =  10.8  KW.,  or  14.47  H.  P. 

If  now  the  flux  could  be  increased  tenfold,  that  is,  to 
40  megawebers,  the  same  total  torque  could  be  obtained 
with  10  amperes.  The  armature  drop  being  0.5  volt, 
the  c.  E.  M.  F.  124.5  volts,  the  speed  would  be  reduced 
to  93.4  revs,  per  minute.  The  work  done  at  the  pulley, 
with  the  same  allowance  for  machine  frictions,  would  be 
1.12  KW.  and  the  energy  absorbed  from  the  mains  by  the 
armature  circuit  1.25  KW. 

In  practice,  however,  the  range  which  is  under  con- 
trol is  a  limited  one ;  the  maximum  limit  of  <^,  being 
set  by  the  rapidly  increasing  reluctivity  of  iron  at  den- 
sities approaching  saturation,  while  the  minimum  limit 
is  established  by  armature  reaction,  which  seriously  dis- 


181 


torts  the  weakened  magnet  flux,  and  may  even  over- 
power it,  causing  violent  sparking  at  the  brushes,  and 
irregular  operation.  In  shunt  machines,  the  ratio  of 
speed  usually  obtainable  by  variation  of  M.  M.  F.  is  about 
25  per  cent.,  while  in  series  machines,  it  may,  under 
favorable  circumstances,  amount  to  doubling  the  speed. 

191.  The  case  of  variable  torque  and  constant  speed 
is  continually  met  with  in  practice  ;  for  example, 
when  a  lathe,  drill,  planer  or  other  large  machine  to'ol, 
has  to  be  driven  at  a  constant  speed  by  an  electric  motor 
under  very  variable  loads.  A  shunt  motor,  connected 
with  constant-potential  mains,  accommodates  itself  very 
closely  to  this  requirement;  for,  neglecting  the  second- 
ary effects  of  armature  reaction,  the  only  diminution  of 
speed  between  light  and  full  loads  is  due  to  the  drop  in 
the  armature  resistance,  representing  a  fall  of  speed  of, 
say,  2  per  cent,  in  a  100  KW.  motor,  and  5  per  cent,  in  a 
1  KW.  motor.  A  series  motor,  however,  is  very  far  from 
complying  with  these  requirements,  since  an  increase  in 
load  automatically  increases  the  M.  M.  F.  of  the  field 
magnets,  thus  increasing  both  the  flux  through  the  arma- 
ture and  the  c.  E.  M.  F.,  allowing  the  speed  to  diminish, 
with  a  further  retardation  due  to  drop  in  the  resistance 
of  the  machine.  Series  motors,  therefore,  are  not  applic- 
able to  cases  where  a  constant  speed  is  automatically  re- 
quired under  conditions  of  variable  load. 

A  compound-wound  generator  is,  however,  capable  of 
being  directly  employed  as  a  compound  motor  without 
any  change  in  its  electrical  connections.  The  series 
winding  here  exerts  a  counter  M.  M.  F.,  (abbreviated, 
c.  M.  M.  F.),  and  thus  diminishes  the  flux  through  the 
armature  at  full  load,  thus  requiring  an  increase  in 


182 


speed,  to  compensate  for  the  drop  in  the  armature  re- 
sistance. Such  machines  are  rarely  needed,  since  the 
regulation  in  speed  obtained  by  shunt  machines  is  usually 
sufficient  for  practical  requirements  and,  moreover,  they 
are  simpler  to  construct, 

192.  Atypical  case  of  variable  torque  and  variable 
speed   is    encountered  in  the    electric  street-car 

motor.  For  the  speed  has  to  be  varied  within  wide 
limits  under  very  varying  conditions  of  torque,  accord- 
ing to  the  number  of  passengers  carried  and  the  gradi- 
ent of  the  track.  Here  the  same  methods  are  adopted, 
as  have  already  been  alluded  to  in  dealing  with  constant 
torque  at  variable  speed  ;  that  is  to  say,  either  resistance 
is  inserted  in  circuit  with  the  armature,  or  the  M.  M.  F. 
of  the  h'eld  magnets  is  varied,  or,  in  some  cases,  both 
means  of  regulation  are  employed.  While  these  afford 
sufficient  regulation  for  street  car  motors,  they  fail  to 
secure  a  perfect  automatic  control  of  speed  under  varied 
conditions  of  torque ;  and,  in  this  respect,  the  electric 
motor  appears  to  least  advantage. 

193.  In  consequence  of  the  reversibility  of  a  gene- 
rator and  motor,  the  same  dynamo  electric  ma- 
chine can,  as  already  observed,  be  employed  in  either 
capacity ;  but  its  output  as  a  generator,  will,  with  con- 
stant excitation  and  speed,  be  always  greater  than  its  out- 
put as  a  motor.   For,  suppose  a  50  KW.  generator  supplying 
a  full-load  current  of  100  amperes  at  500  volts  terminal 
pressure.  With  a  given  excitation  and  speed,  the  frictional 
losses,  magnetic,  electric,  and  mechanical,  will,  perhaps, 
amount  to  5  KW.    These  are  supplied  by  the  engine  when 
the  machine  is  acting  as  a  generator,  with  an  intake  of 


183 


55  KW.  at  the  shaft ;  but,  as  a  motor,  at  the  same  speed 
and  excitation,  the  armature  can  only  maintain  a  current 
strength  of  100  amperes  without  overheating,  while  the 
frictions  must  now  be  supplied  electrically  so  that  the 
output  of  the  machine  will  only  be  about  45  KW. 


INTAKE    (WATTS) 


FIG.  83. 


Curves  showing    Expenditure  of  Power  in  a  750- Watt  Shunt- Wound   Motor  operated 
trom  constant  potential  mains. 

194.     Fig.  83  represents  curves  taken  from  the  test  of 
a  particular  0.75  KW.  shunt  motor,  wound  for  500 
volts.     It  will  be  seen  that  at  full  load,  that  is,  at  a  de- 
livery of  0.75  KW.  at  the  pulley,  the  intake  is  1130  watts, 
representing  a  commercial  efficiency  of  66.4  per  cent. 


184 


Of  this  1130  watts,  90  are  expended  as  iz  /•,  in  the  shunt 
field,  70  as  i*  /•,  in  the  armature,  220  in  mechanical,  eddy, 
and  hysteretic  frictions,  leaving  the  balance  of  750  as 
output. 

SYLLABUS. 

The  condition  of  constant  torque  and  variable  speed 
may  be  obtained  by  inserting  resistance  in  the  circuit  of 
a  shunt  motor,  or  by  commuting  the  fields  of  a  series 
motor. 

The  condition  of  variable  torque  and  constant  speed 
is  very  fairly  met  by  shunt  motors  supplied  from  con- 
stant-potential mains.  It  can  be  still  more  closely  met 
by  compound-wound  motors,  but  is  not  met  by  series 
motors. 

The  condition  of  variable  torque  and  variable  speed 
cannot  at  present  be  automatically  obtained  in  a  single 
motor  operated  from  constant  potential  mains. 

Laboratory  of  Houston  &  Kennelly, 
Philadelphia. 


[Copyright,  1894,  by  THE  ELECTRICAL  ENGINEER.] 
WEEKLY. 


No.  24.  NOVEMBER  24,  1894. 

Electrical   Engineering  Leaflets, 


Prof.  E.  J.  Houston,  Ph.  D. 

AND 

A.  E.  Kennelly,  F.  R.  A.  S. 


INXEf^JYIEDIATTE  CRADE. 

ELKCTRIC  MOTOR. 

(CONTINUOUS    CURRENT   TYPE.) 


195.  Motor  armatures,  like  generator  armatures,  are 
either  of  the  smooth-cored  or  toothed-cored  jfcype. 

In  a  smooth-cored  armature,  the  electrodjnamic  force  is 
largely  exerted  upon  the  substance  of  the  wires,  so  that 
they  are  liable  to  be  dislodged  by  momentarily  powerful 
currents.  In  the  toothed-cored  armature,  the  wires,  em- 
bedded in  iron  grooves,  merely  exert  their  M.  M.  F.,  and 
direct  the  flux  through  the  surrounding  iron,  so  that  the 
electrodynamic  force  is  entirely  exerted,  under  the  dis- 
tribution of ,  between  the  polar  surfaces  and  the  sur- 

8    71 

faces  of  the  iron  armature  projections.     (Sec.  125.) 

196.  Moreover,  the  eddy  currents  that  are  set  up  in 
the  substance  of  the  wires,  when  situated  on  the 

surface  of  a  smooth-cored  armature,  in  passing  through  the 
field  flux,  are  avoided  in  toothed-cored  armatures,  where 
the  flux  is  bodily  guided,  from  side  to  side  of  the  buried 
wires,  through  the  mass  of  the  iron.  For  these  reasons, 

Published  by 
THE  ELECTRICAL  ENGINEER, 

203  Broadway,  New  York,  N.  Y. 

[Entered  asusecond-class  matter  at  the  New  York  }N.  Y.,  Post  Office,  June  14,  1894.] 


186 


motors  with  toothed-cored  armatures  are  rapidly  displac- 
ing smooth-cored  armatures.  Especial  care,  has,  however, 
to  be  taken  in  the  design  of  toothed-cored  armatures,  in 
order  to  prevent  excessive  sparking,  which  is  liable  to  be 
set  up  at  the  brushes,  by  reason  of  the  increased  induct- 
ance of  coils  nearly  surrounded  by  iron.  (Sec.  137,  160 
and  162.) 

197.  Since  the  motor  armature  revolves  under  the 
influence  of  a  distribution  of  flux  between  the 
poles  and  armature,  whereby  the  attractive  force  is  in- 
creased on  one  side  and  diminished  on  the  other,  (Sec. 
125)  the  direction  of  M.  M.  F.  in  a  motor  armature  must 
be  such  as  will  increase,  by  the  flux  it  produces,  the  in- 
tensity at  the  polar  edge  which  the  armature  approaches, 
i.e.,  the  leading  polar  edge,  and  decrease  the  intensity  at 
the  polar  edge  which  it  leaves,  i.e.,  the  following  or 
trailing  polar  edge.  We  have  seen,  however,  that  in  a 
generator,  the  armature  has  to  be  moved  by  mechanical 
force,  against  an  electrodynamic  force ;  and,  consequently, 
the  leading  polar  edge  in  a  dynamo  is  weakened,  while 
the  trailing  polar  edge  has  its  intensity  strengthened  by 
the  armature  reaction  and  M.  M.  F.  The  M.  M.  F.  in  a 
motor  armature,  is,  therefore,  opposed  to  the  direction  of 
M.  M.  F.  in  a  generator  armature,  when  the  direction  of 
rotation  and  the  direction  of  field  M.  M.  F.  are  the  same- 
Tins  is  the  key  to  all  the  relations  existing  between  the 
direction  of  rotation  of  a  machine  when  acting  as  a 
generator  or  as  a  motor. 

Thus,  when  the  direction  of  current  through  the  ma- 
chine, or  the  direction  of  E.  M.  F.  at  the  terminals  of  the 
machine,  remains  the  same,  a  shunt-wound  motor  will 
have  the  same  direction  of  rotation  as  when  employed 


187 


as  a  generator,  while  a  series-wound  machine  will,  on  the 
contrary,  have  the  opposite  direction  of  rotation,  as  a 


MOTOR9 

DIRECTION  Of  j  DIRECTION  OF 

TERMINAL  CURRENT  PRESERVED!    TERMINAL  E.  M.  r.  PRESERVED 

MOTOR         I  I         MOTOR 

8EPARATELY|EXC.  I   SEPARATEl Y|EXC. 


GENERATORS 


£Uc.Rngivttr- 

FIG.  84. 

Showing  Relative  Direction  of  Rotation  in  Generators  and  Motors. 

motor,  that  it  has  when  driven  as  a  generator.     It  also 
follows  that  in  order  to  reverse  the  direction  of  rota- 


188 


tion  of  a  motor  it  is  only  necessary  to  reverse  the 
M.  M.  F.  either  of  the  field  magnets,  or  of  the  armature, 
while,  if  both  be  reversed,  the  direction  of  rotation  will 
remain  unchanged.  For  this  reason  the  mere  reversal  of 
the  terminals  of  any  motor  will  not  alter  its  direction  of 
rotation,  unless  the  field  magnets  are  separately  excited. 
These  conditions  are  exemplified  in  Fig.  84,  where 
the  uppermost  row  of  motors  are  represented  as  sepa- 
rately excited,  the  middle  row  as  shunt-wound,  and  the 
lowest  row,  as  series-wound.  The  large  straight  arrows 
indicate  the  directions  of  the  M.  M.  F.'S  in  fields  and  arma- 
tures, while  the  curved  arrows  indicate  the  direction  of 
rotation.  The  direction  of  E.  M.  F.  in  the  armature  is 
also  in  the  direction  in  which  the  letters  are  marked. 

198.  Motors,  like  generators,  are  capable   of  being 
operated   in  series.     In  practice,  however,  they 

require  either  to  be  mechanically  coupled  together,  so 
that  they  are  forced  to  maintain  the  same  speed,  or,  their 
load  must  be  so  adjusted  that  their  speed  is  automatically 
controlled.  If  this  condition  is  not  complied  with,  the 
motors  are  likely  to  race,  and  thus  give  rise  to  trouble- 
some irregularities  of  speed. 

199.  The  advantages  which  an  electric  motor  possess 
over  other  motors  may  be  enumerated  as  follows. 

(1.)  Facility  of  reversal  of  direction  of  rotation. 

(2.}  Small  size  and  weight  per  kilowatt  (weight  aver- 
age 100  to  180  Ibs.  per  kilowatt  of  output). 

(3.)  Self  governing  power,  or  the  capability  of  auto- 
matic control. 

(4.)  A  high  efficiency. 

(5.)  Rotary  as  opposed  to  reciprocating  motion,  with 
facility  of  operation  and  freedom  from  repairs. 


180 


(6.)  Portability  in  small  sizes,  when  connected  with 
machine  tools,  so  that  a  tool  can  be  brought  to  the  work, 
rather  than  the  work  to  the  tool. 

(7.)  Cleanliness  ;  i.e.,  protection  from  dust,  liquids,  etc. 

(8.)  Convenience  and  efficiency  of  distributing  power 
to  distances  by  means  of  insulated  conductors. 

(9.)  Facility  with  which  the  power  can  be  metered  to 
consumers  and  observed  at  any  moment. 

200.  In  reversing  a  motor,  it  is  merely  necessary,  as 
we  have  seen,  to  reverse  the  M.  M.  F.  either  of  the 
field  magnets,  or  of  the  armature,  and,  in  practice,  it  is 
usually  the  armature  which  is  so  reversed.  It  is  neces- 
sary, however,  to  avoid  making  the  reversal  suddenly, 
unless  resistance  be  temporarily  inserted  in  the  armature 
circuit,  for  the  reason  that  the  momentum  of  the  arma- 
ture, carries  it  in  its  previous  direction,  and  the  E.  M.  F. 
of  the  armature  under  such  conditions  is  no  longer  a 
c.  E.  M.  F.  to  the  circuit,  but  is  a  direct  E.  M.  F.  (contracted 

D.  E.  M.  F.)  tending  to  increase  the  current  strength  that 
will  flow  through  the  armature,  when  connected  with 
the  mains.    Thus,  suppose  a  10  KW.  120- volt  shunt-motor, 
with  100  amperes  full-load  intake,  making  1,000  revolu- 
tions per  minute,  is  connected  to  a  system  of   mains, 
maintaining  a  constant  pressure  of  120  volts.     On  cutting 
off  the  current  from  the  armature,  whose  resistance  may 
be  0.05  ohm,  the  motor  may  take,  say,  sixty  seconds  to 
come  to  rest,  depending  upon  the  amount  of  load  to 
which  it  is  connected.     If,  however,  while  still  running 
at   500   revolutions   per  minute,  the  armature  be  con- 
nected   reversed   to   the    mains,    the   E.   M.    F.  of    the 
armature  will  be  60  volts  in  the  same  direction  as  the 

E.  M.  F.  now  impressed  from  the  mains,  so  that  the  cur- 


190 


rent  strength  which  would  pass  through  the  armature 


according  to   Ohm's  law  would   be          '   ^°  =  3,600 

0.05 

amperes,  or  36  times  greater  than  the  normal,  full-load 
intake.  Of  course  the  inductance  of  the  armature  would 
tend  to  set  up  a  temporary  c.  E.  M.  F.,  independently  of 
the  rotation  of  the  armature,  tending  to  check  this  rush 
of  current,  but  it  is  easy  to  see  that  before  the  momentum 
of  the  armature  can  be  overcome,  and  its  c.  E.  M.  F. 
established  by  acceleration  in  the  opposite  direction,  a 
dangerously  strong  current  may  pass  through  it.  This 
shows  that  either  resistance,  or  inductance,  or  both, 
should  be  inserted  in  the  circuit  of  the  armature  of  a 
motor  when  it  has  to  be  reversed. 

The  same  necessity  for  avoiding  excessive  rush  of  cur- 
rent exists,  although  to  a  smaller  degree,  in  starting  shunt 
motors  from  rest.  A  starting  rheostat,  therefore,  has 
generally  to  be  introduced,  especially  with  large  motors, 
in  order  slowly  to  accelerate  the  armature  and  develop 
its  c.  E.  M.  F.  For  this  reason  series  motors  can  be  more 
safely  started  from  rest  suddenly,  owing  to  the  resistance 
and  inductance  of  the  field  magnet  coils,  which  auto- 
matically check  the  first  rush  of  current  through  the 
machine  before  the  c.  E.  M.  F.  has  had  time  to  develop. 

201.  In  electric  locomotors,  it  is  essential  that  the 
weight  should  be  reduced  as  far  as  possible.  The 
torque  to  be  exerted  by  such  a  motor  varies  with  the 
weight  to  be  moved,  the  friction  of  the  track,  and  the 
gradient.  Fig.  85  represents  a  motor  shaft  M,  geared 
directly  to  the  car  wheel  w.  A  motor  used  in  connection 
with  such  gearing  is  commonly  called  a  single  reduction 
motor. 


191 


If  the  pull,  which  would  have  to  be  exerted  upon  the 
car  in  a  direction  parallel  with  the  track,  be  P,  Ibs.,  due 
to  gradient  and  friction  combined,  then  the  torque  at 
the  axle  of  the  car  TF,  will  be  P  f  Ibs.-feet,  where  /*,  is 
the  radius  of  the  car  wheel  (usually  1.25  feet  in  a  street 
car).  If  j,  be  the  gearing  ratio  of  the  motor  and  wheel, 
so  that  the  motor  makes  J  revolutions  to  each  revolution 
of  the  car  wheel,  the  torque  at  the  motor  shaft  will  be 

P  f 

— f~  Ibs.-ft.     It  is  necessary,  therefore,  that  the  torque 


T  = 


Elec.  Engineer 

FIG.  85. 

Single  Reduction  Gear  between  Street  Car  Motor  and  Car  Wheel. 

(£-^.  cm.  dynes,  which  the  motor  can  exert  with 


the  maximum  permissible  current  *',  amperes,  shall  be 

P  f 

equal  to  —  jL  Ibs.-f  t.  for  the  maximum  gradient  and  fric- 

J 
tion  which  the  car  has  to  overcome.     Since  the  weight 

of  the  motor  adds  to  JP,  it  is  necessary  to  obtain  the 
maximum  amount  of  torque  from  the  motor,  with  the 
smallest  weight  consistent  with  perfect  mechanical  se- 
curity, freedom  from  sparking,  and  other  difficulties. 


192 


This  is  accomplished  in  practice,  for  street  cars  and 
railway  motors,  by  employing  toothed-cored  armatures, 
carbon  brushes,  and  cast  steel  multipolar  Held  magnet 
frames,  so  that  the  maximum  flux  is  obtained  with  the 
minimum  material. 

SYLLABUS. 

Smooth-cored  armatures  are  mechanically  weaker  than 
toothed-cored  armatures. 

The  relative  directions  of  M.  M.  F.  in  armature  and  field, 
for  the  same  direction  of  rotation,  are  reversed  in  motors 
and  generators. 

The  leading  pole  edge  has  its  flux  density  strengthened 
in  a  motor,  and  the  trailing  polar  edge  has  its  flux  density 
strengthened  in  a  generator  by  armature  M.  M.  F. 

It  is  essential  to  introduce  resistance  or  inductance 
into  the  armature  circuit  of  a  motor  which  is  being  re- 
versed or  started  from  rest. 

Laboratory  of  Houston  &  Kennelly, 
Philadelphia. 


[Copyright,  1804,  by  THK  ELKCTRICAI.  ENGINEER. 


WEEKLY. 


No    25  T)TrrTf\raTTK   1     18<U  Price»     '     10Cente- 

*J  X    **'        Subscription,  $3.00. 

Electrical    Engineering   Leaflets, 


Prof.  E.  J.  Houston,  Ph.  D. 

AND 

A.  E.  Kennelly,  F.  R.  A.  S. 


INTERMEDIATE   GRADE. 

RLECTRIC    HBATINQ. 


202.  Tlie  universal  result  of  the  passage  of  an  electric 

Jr  o 

current  through  a  conductor  is  the  generation  of 
heat  in  the  substance  of  the  conductor.  We  have  seen, 
(Sees.  57  to  60)  that  the  passage  of  a  current  of  1  am- 
peres, through  a  resistance  of  R  ohms,  develops  in  the 
conductor  a  c.  E.  M.  F.  of  E  =  /  It  volts  drop.  The 
work  done  by  the  current  against  this  c.  E.  M.  r.  appears 
as  heat  in  the  resistance,  and  is  equal  to  El  joules  per 
second,  or  a  thermal  activity  ofJ?l=  P1  R  watts. 

203.  Careful  determinations  have  been  made  as  to  the 
increase  of  temperature  produced  in  a  given  mass 

of  water  by  a  given  quantity  of  heat.  It  has  been  found 
that  approximately  4.18  joules  of  energy,  expended  as  heat, 
will  raise  the  temperature  of  one  gramme  of  water  from 
3°  C.  to  4°  C.  (and  approximately  1°  C.  at  any  temperature 
between  the  freezing  and  the  boiling  points).  This  unit 
quantity  of  heat  is  called  indifferently  the  lesser  calorie, 
the  gramme  calorie,  the  therm,  or  the 


Published  by 

THE   ELECTRICAL  ENGINEER, 
203  Broadway,  New  York,  N.  Y, 

[Entered  as  second-class  matter  at  the  New  York,  N.  Y.,  Post  Office,  June  14,  1894.] 


194 


degree-centigrade.  A  definite  relation  exists  between 
the  amount  of  heat  required  to  raise  a  gramme  of  water, 
and  a  gramme  of  any  other  substance  through  a  ^iven 
range  of  temperature.  The  amount  of  heat  required 
to  raise  the  temperature  of  one  gramme  mass  of  a  sub- 
stance 1°  C.,  referred  to  that  required  to  raise  the  same 
mass  of  water  as  unity,  is  called  the  yjecijic  heat  of  that 
substance.  Thus,  the  specific  heat  of  copper  is  0.093,  so 
that  the  amount  of  heat  required  to  raise  a  given  mass 
of  copper  through  a  given  range  of  temperature  is  9.3 
per  cent,  of  that  required  to  raise  the  same  mass  of  water 
through  the  same  range  of  temperature. 

If  50  joules  be  expended  uniformly  as  heat  in  one 
pound  of  copper  (453.6  grammes),  each  gramme  will  re- 
ceive yjf  J/g-  =  0.1102  joule.  One  gramme  of  water  would 
be  raised  by  this  amount  of  heat  ^.jf2-  =  0.02637°  C., 
and  one  gramme  of  copper,  having  less  capacity  for  heat, 
would  be  raised  about  ten  times  more,  or  through 
^o||jjL  _  0.2835°  C. 

Electrically  generated  heat  is  commercially  employed 
in  furnaces  for  the  reduction,  refining  and  melting  of 
refractory  metals  and  ores,  for  welding,  for  artificial 
heating  Lnd  for  cooking. 

204.  One  gramme  of  good  coal  is  capable,  when  burned, 
of  producing  33,500  joules.  The  average  effici- 
ency of  a  good  compound  engine  and  boiler,  such  as  are 
employed  in  central  stations  may  practically  be  taken  as 
0.12.  The  average  efficiency  of  the  generators  directly 
connected  with  such  engines  may  be  taken  as  0.9,  and  the 
mean  efficiency  of  the  transmission  systems  of  mains  in 
low  tension  systems,  including  house  wiring  about  0.9. 


195 


The  net  efficiency  of  the  entire  transmission  plant  to  the 
supply  terminals  is,  therefore, 

0.12  X  0.9  X  0.9  =  0.0972, 

so  that  the  amount  of  energy  obtainable  as  heat  from 
one  gramme  of  coal  in  an  electric  heater  at  a  distance  of, 
say,  half  a  mile  from  a  central  station,  is  3,250  joules. 

Although  the  efficiency  of  electrically  distributed 
heating  is,  therefore,  under  practical  conditions,  less  than 
10  per  cent.,  yet,  where  a  small  quantity  of  heat  is  to  be 
employed,  as  in  cooking  ranges,  the  efficiency  may  be 
higher  than  in  the  ordinary  cooking  stove  or  range,  the 
efficiency  of  which  is  probably  at  best  only  6  per  cent., 
for  the  reason  that  in  ordinary  stoves  most  of  the  heat  en- 
ergy passes  out  of  the  chimney  in  warm  air  and  unburnt 
gases.  Moreover,  the  question  of  time  enters  into  the 
relative  advantages,  since  the  electric  stove  can  be  started 
and  stopped  in  operation  immediately,  whereas  a  cooking 
fire  requires  time  both  for  starting  and  for  stopping. 

205.  As  regards  its  construction,  an  electric  heater 
consists  essentially  of  a  resistance   coil,  usually 

of  galvanized  iron,  German  silver,  or  other  suitable  al- 
loy, closely  surrounded  with  thermally  conducting  ma- 
terial in  order  to  communicate  the  heat  developed  in 
this  resistance  to  the  body  of  the  heater.  Or  the  wire 
is  embedded  in  a  mass  of  vitrified  clay,  or  of  enamel.  A 
form  of  such  heater  applied  to  a  teapot  is  shown  in  Fig. 
86,  arranged  for  connections  to  mains  of  50  volts  or  110 
volts  alternating  or  continuous  current  pressures. 

206.  The  losses  of  heat  from  an  electric  furnace  or 
cooking  range  can  be  made  comparatively  small, 

since  the  entire  apparatus  can  be  lined  with  a  thermal 


196 


non-conductor,  such  as  asbestos,  or  an  air  jacket,  and  no 
draught  of  air  has  to  be  supplied  through  the  apparatus 
a&  in  the  case  of  a  combustion  furnace. 

The  amount  of  energy  required  to  heat  up  to  boiling 
point  (100°  C.)  a  IL  S.  gallon  of  water  (3TS6  c.c.)froman 
initial  temperature  of  5°  C.  (41°  F.)  is  approximately  95  X 
3786  X  4.18  =  1,503,000  joules.  The  cost  of  electrical 
energy,  when  supplied  in  small  quantities,  from  the 
street  mains  in  large  cities,  is  usually  about  15  cents  per 
kilowatt-hour  (1,000  watts  during  3,000  seconds,  or 
3,600,000  joules)  so  that  the  cost  of  heating  one  gallon 


FIG.  86. 

Electrically  Heated  Tea  Pot. 

of  water  to  the  boiling  point,  assuming  no  loss  in  the 
heater,  is  about  6J  cents.  The  practical  efficiency  of 
electric  heaters  is  seldom  so  low  as  50  per  cent.,  and  may 
under  specially  favorable  conditions,  reach  95  per  cent., 
so  that  at  an  efficiency  of  TO  per  cent.,  the  cost  of  boiling 
a  gallon  of  water  by  electrically  distributed  heat,  on  a 
small  scale,  amounts  to  about  9  cents.  This  is  much 
more  expensive  than  the  cost  of  the  same  operation  in  a 
combustion  range,  and  the  price  of  the  electric  heater  is 
at  present  also  greater  than  the  price  of  a  gas,  coal,  or 
oil  heater,  but  the  greater  simplicity,  convenience  and 


p.*- 


cleanliness  of  the  electric  heater  for  culinary  purposes  on 
a  small  scale  often  outweighs  its  greater  expense. 

An  electric  car  heater,  supplied  at  500  volts  pressure, 
requires  from  2  to  12  amperes,  according  to  the  size  of 
the  car,  and  the  coldness  of  the  weather. 

207.  An  incandescent  lamp  is  an  instance  of  the  appli- 
cation of  electric  heating  to  the  attainment  of  that 

temperature  in  carbon  at  which  it  emits  luminous  radia- 
tion. Unfortunately  in  order  to  obtain  this  luminous 
radiation  a  very  large  amount  of  non-luminous  radiation 
has  to  be  produced.  Thus,  from  the  glowing  filament  of 
an  ordinary  16  c.  P.  incandescent  lamp,  about  50  joules 
of  thermal  energy  are  emitted  per  second,  and  only  about 
5  per  cent,  of  this  or  2.5  joules  are  emitted  as  luminous 
radiation,  the  remainder  lea  vino:  the  filament  either  in 

J  O 

non-luminous  radiation,  or  in  energy  communicated  to 
the  molecules  of  the  gas  remaining  in  the  globe. 

208.  It  is  a  common  observation  that  wires  carrying 
electric    currents    frequently    become    intensely 

heated.  This  is  becauee  their  resistance  per  cm.,  or  per 
foot,  causes  the  current  they  carry  to  develop  as  $  r,  so 
many  joules  of  heat  per  second  in  their  mass,  that  a  high 
temperature  has  to  be  reached  before  the  losses  of  heat 
by  radiation  and  convection  can  keep  pace  with  the  rate 
of  development.  Until  this  equality  of  output  and  in- 
take are  attained,  the  temperature  of  the  wire  will  be  in- 
creasing. 

209.  A  wire  of  bare  copper  or  iron,  suspended  in  air, 
when  heated  by  a  steady  current,  theoretically  re- 
quires an  indefinitely  long  time  to  acquire  its  maximum 
temperature,  but  practically,  owing  to  the  freedom  with 


198 


which  the  heat  is  carried  away  by  convection  in  the  sur- 
rounding air,  the  full  elevation  of  temperature  is  attained 
in  about  live  minutes.  When,  however,  the  wire  is  in- 
sulated, and  laid  in  wooden  moulding,  as  in  the  case  of 
house  wires,  about  95  per  cent,  of  the  full  increase  in 
temperature  is  usually  attained  in  ten  minutes.  When 
the  wires  are  buried  in  the  ground,  the  temperature  may 
continue  to  rise,  with  large  cables,  for  many  hours, 
but  with  copper  conductors  of  less  than  one  cm.  in 
diameter,  about  95  per  cent,  of  the  maximum  tempera- 
ture elevation  is  usually  attained  in  twenty  minutes  after 
the  application  of  the  current. 

210.  The  amount  of  heat  generated  in  a  wire  for  a 
given  effective  current  strength,  as  measured  by 

a  properly  calibrated  ammeter,  is  the  same,  at  all  ordi- 
nary commercial  frequencies  and  practically  employed 
sizes  of  wire,  whether  the  current  be  continuous  or 
alternating.  In  the  case  of  very  high  frequencies  and 
large  wires,  the  resistance  of  the  circuit  to  alternating 
currents  is  greater  than  that  they  offer  to  continuous 
currents,  owing  to  what  is  termed  the  "  skin  effect"  and, 
therefore,  the  amount  of  heat  developed  in  such  cases  in 
the  wires  would  be  greater  for  alternating  than  for  con- 
tinuous currents. 

211.  The  following  table  gives  the  diameters  of  cop- 
per  wire,    which   will   be   raised   approximately 

20°  C.  (36°  F.)  by  the  current  strengths  shown,  under 
various  conditions,  such,  for  example,  as  in  wooden 
moulding,  or  in  air,  within  doors  or  out  of  doors.  Such 
a  wire  would  be  raised  to  50°  C.  from  an  initial  tem- 
perature of  30°  C.,  and  a  wire  at  a  temperature  of 


199 


50°  C.  can  be  handled  without  discomfort.  Such  a 
diameter  would  therefore  he  safe  to  employ  in  huild- 
ings,  hut  would  not  allow  a  sufficient  margin  of  safety 
for  accidental  overload.  For  this  reason,  the  limiting 
safe  temperature  elevations  and  current  strengths  hitherto 

TABLE  OF  DIAMETERS  OF  COPPER  WIRE,  OF  CONDUCTIVITY  98  PER 
CENT.  MATTHIESSEN'S  STANDARD,  ELEVATED  20°  C.  BY  VARIOUS 
CURRENT  STRENGTHS  ix  AMPERES  (ALTERNATING  OR  CON- 
TINUOUS). 


Effective 
Current 
Strength 
Amperes. 

5 
10 
15 

Covered  Wire 
in  Wooden 
Moulding. 

Bare  Wire  Suspended 
Horizontally  in  Still  Air 
Within  Doors. 

Bright.             Blackened. 

Bare  Wire  Suspended 
Horizontally  in  Calm 
•    Weather  Out  of  Doors. 

Bright.            Blackened. 

Inches. 

O.O2O 

0.036 
0.052 

Inches. 
0.015 
o  030 
0.045 

Inches. 
0.014 
0.028 
0.042 

Inches. 

O.O1I 
0.022 
0.032 

Inches. 

O.OIO 
O.O2O 

0.030 

20 

25 
30 

0.069 
0.085 

O.IOO 

0.060 
0.075 
0.090 

0.057                    0.042 
0.068                    0.052 
0.080                    0.06  1 

0.039 
0.049 
0.058 

35 
40 
45 

0.114 
0.127 
0.140 

0.103 
0.115 
0.128 

0.092 
0.105 
0.117 

0.070 
0079 
0.087 

0.066 
0.074 
0.082 

So 
60 
70 

0.152 
0.175 
0.197 

0.140 
0.168 
0.190 

0.130 
0.152 
0.171 

0.094 
0.108 
0.122 

0.089 
0.103 
0.116 

80 
90 

100 

o  218 
0.236 

o.'54 

0.212 

0.235 
0257 

0.192 

O.2IO 
0.227 

0-134 
0.146 
0-157 

0.128 
0.140 
0.151 

125 
150 
J75 

0.292 
0.326 
0-357 

0.307 

0.365 
0.410 

0.265 
0.308 

0-347 

O.l83 
0.210 
0.234 

°-*75 

0.202 

0.227 

200 
250 
300 

0.386 
0.440 

0.450 
0.520 
0.615 

0.385 
o-455 
0.518 

0.256 
0.299 
0-339 

0.248 
0.290 
o-33oj 

400 
500 
600 

0.765 

o  910 

0.640 
0.750 
0.857 

0.418 
-     0.488 
0.550 

0.406 
0.471 
o-533 

700 
800 
900 

• 

:::: 

0.958 

0.611 
0.671 
0.717 

0-593 
0.650 
0.693 

1000 

.... 

.... 

.   .,••*"   -,* 

0.782 

o-745 

200 


adopted  by  fire  insurance  authorities  are  considerably 
lower,  corresponding  to  about  10°  C.  temperature  ele- 
vation at  full  load  and  with  a  reduction  of  about  33  per 
cent,  in  current  strength. 

212.  The  sudden  heating  effect  of  excessive  currents 
is  practically  employed  in  fuse  wires,  which  are 
always  connected  in  circuits  in  order  to  protect  the  wires 
or  apparatus  in  those  circuits  from  excessive  current 
strength.  These  wires  or  strips  are  usually  composed  of 
alloys  of  tin  and  lead,  so  as  to  possess  a  high  resistivity 
and  a  low  melting  point.  A  high  resistivity  enables  an 
amount  of  heat  to  be  generated  in  them  per  square 
centimetre  of  cross-section,  sufficiently  great  to  produce 
the  fusing  effect  desired. 

A  safety  fuse  is  generally  so  proportioned  as  to  melt 
at  50  per  cent,  overload.  Thus,  when  a  full-load  cur- 
rent which  a  wire  has  to  carry  is  50  amperes,  it  is  usual 
to  place  in  circuit  with  it  a  safety  fuse  of  75  amperes 
fusing  current. 

•  SYLLABUS. 

The  passage  of  an  electric  current  against  thec.E.M.F. 
established  in  a  resistance,  does  work  at  the  rate  of  iz  r 
watts  or  joules  per  second,  which  appears  as  heat  in  the 
resistance. 

The  same  amount  of  heat  is  practically  produced  in 
the  same  wire  by  a  given  effective  current  strength, 
whether  the  current  be  alternating  or  continuous. 

For  small  cooking  stoves  electric  heating  is  more  effi- 
cient and  more  convenient,  although  at  the  present  time 
more  costly. 

Laboratory  of  Houston  &  Kennelly, 
Philadelphia, 


[Copyright,  1894,  by  THE  ELECTRICAL  ENGINEER.] 
WEEKLY. 

No.  26.  DECEMBER  8,  1894.       ^np'tion!  »M. 

Electrical   Engineering   Leaflets, 


Prof.  E.  J.  Houston,  Ph.  D. 

AND 

A.  E.  Kennelly,  F.  R.  A.  S. 


INCANDESCENT 


213.  An  incandescent  lamp  consists  essentially  of  a 
filament  of  refractory  material,  almost  invariably 

carbon,  electrically  heated  to  incandescence  and  pro- 
tected from  oxidation  by  enclosure  within  a  sealed  and 
exhausted  glass  globe. 

The  manufacture  of  an  incandescent  lamp  consists  es- 
sentially of  the  following  steps, 

(1.)  The  manufacture  of  the  filament. 

(2.)  The  connection  with  the  leading-in  wires  and  sup- 
port within  the  glass  globe. 

(3.)  The  obtaining  of  the  proper  vacuum. 

214.  Different  processes  have  been  devised  for  ob- 
taining  the    material   of    the    carbon    filament. 

Briefly,  they  are  as  follows :  A  suitable  carbonizable 
material,  after  being  properly  shaped,  is  subjected  to  car- 
bonization under  the  prolonged  action  of  a  high  temper- 
ature while  out  of  contact  with  air.  The  time  required 
for  the  heating  and  cooling  of  a  set  of  filaments  of  carbon 

Published  by 

THE  ELECTRICAL  ENGINEER, 
203  Broadway,  New  York,  N.  Y, 

[Entered  as  second-class  matter  at  the  New  York,  N.  Y.,  Post  Office,  June  14,  1894.] 


202 


in  a  furnace  may  be  two  days.  The  following  materials 
have  been  employed  for  this  purpose ;  viz.,  loosely  spun 
cotton  thread  cleansed  from  grease  either  in  its  natural 
state  or  parchmentized  by  the  action  of  sulphuric  acid; 
selected  bamboo  fibre  ;  celluloid,  silk  thread  and  various 
carbonacous  liquids  or  carbonacous  pastes.  There  are  thus 
obtained  slender  threads  or  filaments  of  the  required  di- 
mensions, possessing  sufficient  rigidity  and  elasticity  to 
stand  mounting,  transportation  and  vibration  while  in  use. 
For  actual  use  the  carbon  filament  must  be  homogeneous 
in  structure,  and  must  possess  uniform  electrical  resist- 
ance per  unit  length,  since,  otherwise,  when  heated  by 
the  current,  it  would  glow  unequally  and  only  parts 
could  be  safely  heated  to  the  temperature  of  illumina- 
tion. 

215.  The  leading-in  wires,  that  is,  the  wires  that  carry 
the  incandescing  current  to  the  filament,  are  al- 
most invariably  made  of  platinum,  since  the  similarity 
in  the  coefficients  of  expansion  of  platinum  and  glass, 
enables  the  platinum  wire  to  be  fused  into  the  glass  and 
subjected  to  fairly  wide  changes  of  temperature  without 
endangering  the  vacuum  through  subsequent  expansions 
and  contractions.  Moreover,  the  high  melting  point  of 
platinum,  permits  the  glass  to  be  fused  around  it  without 
being  destroyed. 

Various  methods  are  employed  for  connecting  the 
ends  of  the  filament  with  the  ends  of  the  leading-in 
wires.  In  all  cases,  however,  two  requirements  exist; 
namely,  that  the  connection  is  electrically  good,  and 
that  the  resistance  of  the  ends  of  the  filament,  where 
they  are  connected  to  the  wires,  is  such  as  to  prevent 
excessively  high  temperature  being  attained  at  the  joints. 


203 


This  is  avoided  either  by  lowering  the  resistance  of  the 
joint  or  by  the  thermal  conductance  of  the  wires. 

216.  The  mounted  filament  is  now  subjected  to  a 
process  called  t\\Q  flashing  process,  for  the  purpose 

of  producing  a  more  durable  carbon  surface.  The  flash- 
ing process  is  conducted  briefly  as  follows:  the  mounted 
filament  is  electrically  heated  to  a  dull  red,  while  sur- 
rounded by  a  hydro-carbon  vapor,  and  the  current 
strength  is  gradually  increased.  At  first,  the  carbon 
may  glow  unequally,  the  points  of  highest  resistance 
first  reaching  incandescence  and  receiving  a  deposit  of 
carbon,  which  tends  to  reduce  the  resistance  at  that  spot. 
The  current  then  being  increased,  the  deposit  is  caused  to 
be  formed  gradually  over  the  entire  surface  of  the  fila- 
ment, thereby  rendering  its  resistance  uniform  through- 
out, until,  when  a  certain  maximum  current  strength  is 
passing,  the  filament  glows  uniformly  and  emits  the  re- 
quired candle  power.  The  entire  flashing  process  re- 
quires but  a  few  seconds  to  complete.  A  flashed  filament 
is  thus  provided  with  a  coating  of  carbon,  which  possesses 
greater  durability  at  high  temperatures  than  unflashed 
carbon. 

217.  The  mounted  filament  is  now  introduced  into 
the  lamp,  and  its  glass  support  P,  (Fig.  87)  fused 

to  the  lamp  bulb,  thus  hermetically  sealing  the  lower 
end  of  the  lamp  chamber. 

The  lamp  chamber  is  now  placed  in  connection  with 
a  vacuum  pump,  by  means  of  an  open  glass  tube  at 
the  top  of  the  globe,  and  exhausted.  The  best  results 
in  regard  to  efficiency  and  durability  are  obtained 
with  the  highest  vacuum.  In  order  to  obtain  this,  it  is 


necessary  to  heat  the  body  of  the  lamp  chamber  in  order 
to  drive  off  the  film  of  condensed  gas  or  air  adhering  to 
the  glass,  and  also  to  heat  the  filament  to  drive  out  the 
occluded  gas.  Both  of  these  heatings  are  generally  ob- 
tained by  submitting  the  lamp  to  the  action  of  the  cur- 
rent during  the  final  stages  of  exhaustion.  The  lamp  is 
then  hermetically  sealed  at  its  tip  T,  by  the  fusion  of  the 
glass  and  removed  from  the  pump. 

The  platinum  leading-in  wire  is  made  as  short  as  pos- 
sible and  occupies  the  length  jj,  jj  as  shown,  the  free 
ends  of  the  platinum  wires,  being  welded  to  copper 


FIG.  87. 

16  c.  P.  Incandescent  Lamp. 

wires.  These  copper  wires  are  soldered  to  the  base  A, 
one  wire  being  connected  to  the  external  brass  screw 
shell  A,  and  the  other  to  the  brass  base  B,  these  two 
parts  being  insulated  from  each  other  by  plaster  of  Paris. 
Different  forms  are  given  to  the  lamp  bases,  ascording 
to  the  lighting  system  used. 

The  bases  shown  in  Fig.  88  are  in  common  use.  A,  is 
a  standard  Edison  base  ;  B,  is  a  Thomson-Houston  or  old 
Sawyer-Man  base  ;  c,  is  a  Westinghouse,  or  new  Sawyer- 
Man  base ;  D,  is  a  United  States  base.  In  all  cases  the 
arrangement  is  such  that  merely  placing  the  lamp  in  a 
socket  connects  it  with  the  mains. 


205 


218.     The  amount  of  activity  absorbed  by  an  incan- 
descent lamp  in  watts,  when  connected  to  supply 

mains  at  a  steady  pressure  of  E  volts,  is  — -  watts,  where 

R  is  the  resistance  of  the  lamp  when  hot.  The  energy 
expended  by  the  lamp  will  be  Sp  watts,  where  $,  is  the 
surface  of  the  filament  in  square  inches,  or  square  centi- 
meters, and  J9,  the  corresponding  emissivity  in  watts  per 
square  inch,  or  per  square  centimetre,  for  the  particular 


FIG.  88. 

Standard  Forms  of  Lamp  Base.     A,  Edison;  B,  Thomson- Houston;  c,  Westinghouse; 
D,  United  States. 

temperature  at  which  the  lamp  is  to  be  operated.     Con- 
sequently, since  the  intake  must  be  equal  to  the  output, 
E* 
R 

The  surface  area  provided  for  the  filament  /$,  is  deter- 
mined from  the  number  of  candles  which  the  lamp  has  to 
supply,  the  quality  of  the  carbon  surface,  and  the  tempera- 
ture at  which  the  lamp  is  to  be  operated ;  so  that,  when 
the  quality  and  temperature  are  fixed,  the  surface  in- 
creases with  the  candle-power.  The  cross  section  and 
length  of  the  filament  must,  therefore,  be  so  chosen  for 
the  resistivity  of  the  material,  that  the  required  resistance 
R,  and  the  required  surface  S9  are  obtained.  The  resist- 
ivity of  lamp  filaments  is  usually  about  O.OOi  ohm,  except 
for  -flashed  carbon  ;  which  has  a  lower  resistivity. 


206 


The  temperature  coefficient  of  filament  carbons  is  neg- 
ative (Sec.  32),  but  that  of  flashed  carbon  is  positive  at 
incandescent  temperatures.  A  heavily  flashed  filament 
may  therefore  increase  in  resistance  as  its  temperature  is 
increased  beyond  that  of  normal  incandescence. 

219.  When  a  very  feeble  current  is  sent  through  a 
lamp,  it  emits  no  light,  all  its  radiation  being  non- 
luminous  or  heat  radiation.     The  activity  which  is  ex- 
pended in  the  lamp,  in  such  cases,  is  wasted  so  far  as 
illumination  is  concerned.  Increasing  the  current  strength 
through  the  lamp,  the  temperature  of  the  filament  increases 
and  the  lamp  commences  to  glow,  the  useful  proportion 
of  luminous  radiation  increasing  rapidly  with  the  tem- 
perature.   The  incandescent  lamp,  therefore,  produces  its 
greatest  illumination  efficiency,   with   the  highest  safe 
temperature  at  which  it  is  possible  to  operate  it. 

220.  The  candle-power  of  a  light  is  measured  in  dif- 
ferent units  in   different  countries.     In  America 

and  Great  Britain,  the  standard  candle  is  employed.  In 
France,  the  carcel  lamp,  the  Violle,  or  molten  platinum 
lamp,  and  a  candle  called  the  bougie  decimal,  which  is 
J^th  of.  the  Yiolle,  are  employed.  In  Germany,  the 
Hefner-Alteneck  lamp,  burning  amyl  acetate,  is  em- 
ployed. 

221.  The  efficiency  of  an  incandescent  lamp  is  fre- 
quently incorrectly  stated  as  being  equal  to  the 

number  of  watts  absorbed  by  the  lamp,  divided  by  the 
number  of  candles ;  that  is,  the  number  of  watts  per  candle, 
so  that  the  efficiency  thus  stated  would  be  higher,  the 
greater  the  waste  in  the  lamp.  The  more  correct  expres- 
sion is  the  number  of  candles  divided  by  the  number  of 
watts  absorbed,  or  the  candles  per  watt. 


207 


Lamps  are  usually  operated  at  one  01  three  efficiencies; 
namely,  at 

3 \^  candle  per  watt ;  so  that  a  16  candle-power  lamp 
absorbs  50  watts. 

j1^  candle  per  watt;  so  that  a  16  candle-power  lamp 
absorbs  57.6  watts. 

J  candle  per  watt ;  so  that  a  16  candle-power  lamp 
absorbs  64  watts. 

Under  ordinary  circumstances,  therefore,  the  advan- 
tage of  an  efficiency  of  F1r  candle  per  watt  amounts  in 
economy  of  energy  22J  per  cent,  over  a  lamp  of  J  candle 
per  watt.  For  the  same  quality  of  carbon  employed, 
the  high  efficiency  lamp  has  to  be  operated  at  a  higher 
temperature  and  its  lifetime  is,  therefore,  consider- 
ably reduced.  Thus,  a  lamp,  which  is  operated  at  an 
efficiency  of  J  candle  per  watt,  may  last,  on  an  average, 
say  5,000  hours ;  while  the  same  lamp,  if  operated  at 
such  an  increase  in  voltage  as  will  cause  its  temperature 
and  candle-power  to  rise,  and  its  efficiency  to  reach  -fa 
candle  per  watt,  may  last,  on  an  average,  only  800  hours, 
owing  to  the  more  rapid  disintegration  of  the  filament 
at  the  higher  temperature. 

At  an  efficiency  of  J  candle  per  watt,  incandescent 
filaments  give  a  luminous  intensity  of  from  100  to  150 
candles  per  sq.  in.  of  surface  (15.5  to  23.25  candles  per 
sq.  cm".).  At  J  candle  per  watt  the  intensity  varies  be- 
tween 160  and  270  candles  per  sq.  in.  (24.5  and  42  candles 
per  sq.  cm.) 

222.     Incandescent  lamps  are  made  in  sizes  ranging 

from  J  candle-power  up   to   100   candle-power. 

Yery  small  lamps  are  operated  at  a  low  efficiency,  say  -J 

candle  per  watt,  owing  partly  to  the  rapid  conduction 


208 


of  heat  from  the  short  filaments  by  the  leading-in  wires. 
Such  small  lamps  are  only  operated  by  batteries. 

The  common  candle-powers  for  incandescent  lamps  are 
8,  10,  16,  20,  32,  50,  and  100  candles. 

Single-filament  incandescent  lamps,  are  made  for  opera- 
tion on  pressures  varying  from  50  to  220  volts.  The 
lamps  of  highest  pressure  are  at  present  only  employed 
at  somewhat  lower  efficiency.  , 

An  incandescent  lamp  gives  the  same  amount  of  light 
when  connected  to  alternating  or  continuous  current 
mains  at  the  same  effective  pressure. 

SYLLABUS. 

The  filament  of  an  incandescent  lamp  is  made  of  car- 
bon enclosed  in  an  exhausted  glass  globe  and  heated  elec- 
trically to  incandescence. 

By  flashing  a  carbon  filament,  its  surface  is  rendered 
homogeneous  and  durable. 

In  exhausting  an  incandescent  lamp  both  the  filament 
and  bulb  are  heated  to  aid  in  expelling  condensed  or  oc- 
cluded gas. 

The  emissivity  of  the  surface  of  a  carbon  filament  at 
an  efficiency  of  -^T  candles  per  watt  varies  between  .  24 
and  42  candles  per  sq.  cm. 

The  efficiency  of  a  lamp  is  commonly  expressed  in  watts 
per  candle,  but  should  be  expressed  in  candles  per  watt. 

Laboratory  of  Houston  &  Kennelly, 
Philadelphia. 


[Copyright,  1894,  by  THE  ELECTRICAL  ENGINEER.] 


WEEKLY. 


No    97  DTrnwMTVFR  1^1  8Q4-          Price»     '    10  Cents' 

Li),  1    W.        Subscription,  $3.00. 

Electrical    Engineering   Leaflets, 


Prof.  E.  J.  Houston,  Ph.  D. 

AND 

A.  E.  Kennelly,  F.  R.  A.  S. 


INTERMEDIATE   GRADE. 

INCANDESCENT    LIGHTINQ. 


223.  By  the  illumination  of  a  body  is  meant  the 
amount  of  light  received  by  it  per  unit  of  sur- 
face area.  Calling  the  body  giving  the  light  the  illumi- 
nating body,  the  body  receiving  the  light,  the  illuminated 
body,  and  the  amount  of  light  received  per  unit  of  area, 
the  illumination,  or  the  intensity  of  incident  light,  then,  if 
the  light  given  by  the  illuminating  body  is  assumed  to  be 
concentrated  at  a  point,  the  intensity  of  light  received  by 
the  illuminated  body  will  be  inversely  proportional  to  the 
square  of  its  distance  from  the  illuminating  body.  No 
name  has  yet  been  generally  adopted  for  the  unit  of  illu- 
mination, although  the  terms  carcel-metre  and  candle-foot, 
have  been  proposed  and  are  in  limited  use.  The  illumi- 
nation of  one  car  eel-metre  is  the  amount  of  illumination 
received  by  a  surface  perpendicular  to  the  rays  of  light 
from  one  carcel,  at  a  distance  of  one  metre.  One  carcel- 
metre  =  0.883  candle-foot,  so  that  one  candle  produces 
at  a  distance  of  a  foot  an  illumination  13  per  cent,  in 

Published  by 

THE   ELECTRICAL   ENGINEER, 
203  Broadway,  New  York,  N.  Y. 

[Entered  as  second-class  matter  at  the  New  York,  N.  Y.,  Post  Office,  June  14,  1894.] 


210 


excess  of  that  produced  by  a  carcel  at  a  distance  of  a 
metre.  The  illumination  of  one  carrel-metre  upon  the 
surface  of  a  printed  page,  is  amply  sufficient  for  the  pur- 
poses of  reading  by  the  normal  eye. 

In  practice  it  is  difficult  to  determine  by  calculation 
the  illumination  at  any  surface  in  a  room,  when  the 
position,  candle-power  and  number  of  incandescent 
lamps  in  the  room  are  assigned,  owing  to  the  influence 
produced  by  the  nature  and  color  of  the  walls,  ceilings, 
draperies  and  furniture  in  the  room.  The  amount  of  light 
reflected  or  diffused  from  the  surface  of  light  colored 
wall-paper  is  frequently  40  per  cent.,  the  remaining  sixty 
per  cent,  being  absorbed  by  the  paper.  Usually,  however, 
it  is  found  that  ^  candle  per  square  foot  of  floor  space 
(3.6  candles  per  square  metre)  distributed  in  electric 
lamps,  provides  ample  illumination  for  reading  rooms, 
representing  one  16  candle-power  lamp  for  every  50  square 
feet  of  floor  surface.  In  ordinary  ro  ;ms,  not  devoted 
exclusively  to  reading,  half  this  amount  of  illumination, 
or  one  16  candle-power  lamp  to  100  square  feet  of  floor 
space  will  suffice. 

224.  The  efiect  of  continued  use  of  an  incandescent 
lamp  even  when  the  pressure  supplied  to  it  does 
not  exceed  that  for  which  the  lamp  was  designed,  is  to 
decrease  the  diameter  of  the  filament;  and,  consequently, 
to  increase  its  resistance.  This  decrease  in  the  thickness 
of  the  filament  is  due  to  the  wasting  away  of  its  surface 
layers. 

A  lamp  which  when  first  used  has  an  efficiency  of,  say 
J  candle  per  watt,  gradually  decreases  in  efficiency,  un- 
til, after  1000  hours,  its  efficiency  may  only  be  £  candle 
per  watt. 


211 


225.  Lamps  are  generally  guaranteed  to  last  600 
hours  under  conditions  of  normal  operation,  that  is, 
when  not  operated  above  the  pressure  for  which  they  were 
designed.  Their  average  life  depends  entirely  upon  the 
temperature  at  which  they  are  worked.  At  an  exceed- 
ingly high  temperature,  perhaps  1380°  C.,  when  the  in- 
terior of  the  lamp  appears  bluish,  the  brilliancy  of 
incandescence  is  very  marked,  and  the  efficiency,  meas- 
ured in  candles  per  watt,  very  high,  say  1.25  candles  per 
watt,  but  their  life  may  be  only  one  hour.  On  the  other 
hand,  a  lamp  operated  at  a  dull  red,  gives  very  little 
light  and  has  a  low  temperature,  perhaps  1200°  C.,  with 
an  efficiency  of,  perhaps,  ^  candle  per  watt,  but  will 
continue  to  burn  at  this  rate  almost  indefinitely.  The 
average  life  time  of  a  properly  constructed  lamp,  there- 
fore, depends  upon  the  temperature  at  which  it  is  de- 
signed to  operate  initially,  that  is  upon  its  normal 
efficiency,  and  a  high  efficiency  lamp  is  only  attained, 
other  things  being  equal,  at  the  expense  of  its  life  time. 

A  lamp  which  has  been  burning  at  a  gradually  decreas- 
ing efficiency,  for,  say  1000  hours,  may  at  length  have 
its  chamber  become  so  blackened  by  continued  deposi- 
tion of  carbon,  that  from  this  cause,  together  witii  the 
concomitant  reduction  in  the  emission  of  light,  it  may 
become  unserviceable,  and  it  may  be  cheaper  to  destroy 
the  lamp  and  replace  it  by  a  new  one  than  to  continue  its 
use.  The  exact  period  at  which  it  becomes  economical 
to  do  this  is  very  difficult  to  decide.  In  large  central 
station  practice,  it  is  usually  considered  that  lamps  are 
properly  operated  in  regard  to  their  pressure,  when  the 
cost  of  lamp  renewals  amounts  to  15  per  cent,  of  the  total 
operating  expenses ;  but,  in  a  single  installation,  no  fixed 


212 


rule  can  be  laid  down,  since,  if  ample  light  lias  been  pro- 
vided at  the  outset  a  fairly  marked  decrease  in  the  light 
attending  increased  age  of  the  lamps,  will  not  be  ob- 
jectionable and  it  may  be  economical  to  retain  lamps  for 
a  very  long  period. 

226.  It  has  been  established  by  actual  practice  up  to 
the  present  time,  that  where  a  densely  lighted 

district  can  receive  incandescent  lighting  distribution 
from  a  central  station  near  its  centre,  it  is  more  economi- 
cal to  employ  low-tension,  multiple-connected  systems, 
on  either  the  two-wire,  three-wire,  or  five-wire  plant, 
but,  on  the  contrary,  where  the  lighting  is  scattered  over 
a  large  district,  and  where  the  distances  to  be  covered 
are  great,  the  cost  of  conductors  required  for  low-ten- 
sion lighting  becomes  excessive,  and  a  high-tension  sys- 
tem imperative.  For  street  incandescent  lighting,  under 
such  circumstances,  the  lamps  can  be  connected  to  the 
line  in  series,  like  series  arc  circuits  ;  but  where  interior 
lighting  has  to  be  provided,  it  is  almost  always  carried 
out  by  the  use  of  alternating  current,  high-tension,  sys- 
tems, with  step-down,  local,  transformers.  In  some  cases 
incandescent  lamps  are  required  to  be  operated  in  arc 
circuits,  in  which  case  they  have  to  carry  a  current 
strength  of  about  10  amperes.  For  the  same  output 
and  efficiency,  the  pressure  at  such  a  lamp,  of  say  16 
candles,  would  be  approximately  six  volts,  and  its  fila- 
ment would  have,  when  hot,  a  resistance  of  about  ^  ohm? 
requiring,  therefore,  a  short  thick  carbon. 

227.  With  series-connected  lamps,  in  order  that  the 
failure  of  a  single  lamp  shall  not  open  the  entire 

circuit,  it  is  necessary  that  some  device  should  be  pro- 


213 


vided  for  maintaining  the  continuity  of  the  circuit  when 
any  lamp  fails.  This  is  usually  obtained  by  means  of  a 
cut-out,  which  short  circuits  the  lamp  as  soon  as  the  pres- 
sure at  the  lamp  terminals  exceeds  that  required  for  its 
ordinary  operation. 

Since  an  ordinary  gas-burner  of  16  candle-power  con- 
sumes about  5  cubic  feet  of  gas  per  hour,  the  cost  of 
supplying  light  electrically  in  a  16  candle-power  incan- 
descent lamp,  including  the  cost  of  renewing  the  lamp 
when  it  becomes  broken  or  disabled  by  age,  lias  to  be 


FIG.  89. 

Forms  of  Ratchet  Snap  Switches. 

compared  with  the  cost  of  5  cubic  feet  of  gas  in  order 
to  determine  the  relative  economy  of  the  two  illumi- 
nants,  apart  from  all  considerations  of  safety,  health  and 
convenience.  Thus  the  incandescent  lighting  rate  of  £ 
cent  per  16  c.  P.  lamp-hour,  including  lamp  renewals,  is 
equivalent  to  gas  at  $1.50  per  thousand  cubic  feet. 

228.     Incandescent   light  switches,  for  multiple-con- 
nected incandescent  lamps,  are  either  single-pole 
or  double-pole.     A  single-pole  switch  is  similar  in  con- 


214 


nection  to  the  key  of  the  ordinary  lamp  socket,  and 
merely  breaks  the  circuit  at  one  point.  A  double-pole 
switch  interrupts  the  circuit  of  the  lamp  or  lamps  it  con- 
trols, on  each  side  of  the  circuit ;  i.e.,  breaks  both  the 
positive  and  negative  connections.  Forms  of  ratchet 
snap  switches  are  shown  in  Fig.  89.  A,  is  a  25-ampere 
switch.  B  and  c,  are  10-ampere  switches,  that  is  to  say, 
the  maximum  current  they  are  intended  to  carry  is  10 
amperes. 

229.  In  all  multiple  incandescent  systems  the  pres- 
sure has  to  be  maintained  as  nearly  uniform  as 

possible.  Consequently,  the  drop  of  pressure  in  the 
supply  main  has  to  be  kept  within  narrow  limits. 

Specifications  for  the  sizes  of  wire  employed  in  wir- 
ing buildings  for  incandescent  lighting,  usually  require 
that  the  drop  in  pressure  shall  not  exceed  3  per  cent,  of 
the  pressure  at  the  generator  terminals,  if  the  building 
contains  its  own  generator,  or  of  the  pressure  at  the  en- 
trance mains,  if  the  building  is  supplied  from  street  con- 
ductors. In  large  buildings,  with  many  lamps  and  long 
distances  to  be  covered  by  wiring,  the  limit  of  drop  may 
be  increased  to  5  per  cent.,  at  full  load.  Under  these 
circumstances,  if  the  lamps  be  supplied  for  the  mean 
pressure  in  the  building,  the  most  distant  lamps  will  be 
operated  at  about  2^  per  cent,  below,  while  the  lamps 
nearest  to  the  generator  or  street  mains  will  be  about  2£ 
per  cent,  above  pressure. 

230.  Various  devices  have  been   employed  in  order 
to  reduce  the  candle-power  of  a  lamp  by  turning 

it  down,  as  can  be  done  with  any  ordinary  gas  burner. 
All  such  methods  have,  however,  hitherto  required  that 


215 


a  resistance,  or  its  equivalent,   be  introduced  into  the 
circuit  of  the  lamp,   thereby  reducing  its  current  and 


FIG.  90. 

Theatre  Dimmer. 


temperature.  Under  these  circumstances,  while  the 
lamp  gives  less  light,  its  efficiency  is  lowered  and  the 
the  color  of  its  light  is  much  duller,  whereas  the  color 
of  the  gas  flame  is  scarcely  affected  by  turning  it  down. 


FIG.  91. 

Ordinary  Forms  of  Miniature  Lamps. 


Large  resistances  are  sometimes  employed  in  reducing 
the  candle-power  of  lamps  for  scenic  effects  in  theatres. 


216 


During  the  time  that  the  light  is  reduced,  the  activity  of 
the  lamps  is  always  reduced  in  much  smaller  degree,  so 
that  the  efficiency  of  the  remaining  incandescence  is 
very  low.  Fig.  90  represents  a  form  of  theatre  dimmer, 
about  one  foot  square,  consisting  of  a  wire  resistance 
embedded  in  enamel  laid  upon  a  metal  plate. 

Fig.  91  represents  the  ordinary  forms  of  miniature  in- 
candescent lamps  as  employed  for  microscopical  and 
surgical  purposes. 

SYLLABUS. 

The  illumination  of  a  body  is  the  amount  of  light  it 
receives  per  unit  of  surface  area. 

The  efficiency  and  candle-power  of  a  lamp  diminishes 
with  use,  owing  to  the  reduction  of  the  cross-section  of 
the  filament,  the  change  in  the  surface  of  the  filament, 
and  the  blackening  of  the  globe. 

The  usual  amount  of  drop  permitted  in  incandescent 
street  mains  is  five  per  cent,  of  the  central  station  pres- 
sure, and  the  usual  amount  of  drop  in  the  wiring  of 
buildings  varies  from  two  to  five  per  cent.,  according  to 
the  size  of  the  building. 

Laboratory  of  Houston  &  Kennelly, 
Philadelphia. 


Copyright,  1894,  by  THE  ELECTRICAL  ENGINEER.] 


WEEKLY. 


NY>    98  D^n™™™  99    1KQ4-         Price»    '    10  Cents< 

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Electrical    Engineering   Leaflets, 


Prof.  E.  J.  Houston,  Ph.  D. 

AND 

A.  E.  Kennelly,  F.  R.  A.  S. 


IWTEWJYIEDIATE 
ARC 


231.  The  carbon  voltaic  arc  is  produced  by  sending  a 
sufficiently  strong  current  through   two   carbon 

electrodes,  which  are  at  first  in  loose  contact  and  are 
afterwards  gradually  separated  to  a  short  distance.  When 
the  current  is  sufficiently  powerful,  the  space  between 
the  ends  of  the  electrodes  is  filled  with  incandescent  car- 
bon vapor,  in  the  form  of  a  luminous  bow,  which,  from 
its  shape,  is  called  the  voltaic  arc.  The  carbon  vapor  is 
disengaged  mainly  from  the  end  of  the  positive  carbon, 
which  soon  thereby  becomes  hollowed  out  in  the  form 
of  a  minute  crater. 

232.  To  produce  and  maintain  the  voltaic  arc,  a  cer- 
tain electric  activity  is  necessary,  depending  upon 

the  distance  between  the  electrodes,  their  magnitude, 
position  and  incandescent  surfaces.  The  average  arc,  as 
employed  in  the  U.  S.  for  commercial  lighting,  requires 
a  current  of  about  10  amperes,  and  a  total  pressure  at 
lamp  terminals  of  about  45  volts,  representing  an  activity 
of  450  watts.  Of  the  total  c.  E.  M.  F.  of  45  volts,  from 

Published  by 

THE  ELECTRICAL   ENGINEER, 
303  Broadway,  New  York,  N.  Y, 

[Entered  as  second-class  matter  at  the  New  York,  N.  Y.,  Post  Office,  June  14,  1894.] 


218 


38  to  40  volts  are  developed  at  the  surface  of  the  posi- 
tive electrode,  about  five  volts  are  developed  in  the  arc 
itself,  and  the  remainder  are  developed  in  the  resistance 
of  the  lamp  mechanism.  The  activity  at  the  surface  of 
the  positive  carbon  is,  therefore,  from  380  to  400  watts, 
and  this  is  the  principal  source  of  radiant  energy. 

233.  In  an  electrolytic  cell,  a  c.  E.  M.  F.  is  set  up  at 
the  surface  of  the  electrode,  and  the  value  of  this 

c.  E.  M.  F.  is  practically  independent  of  the  c.  E.  M.  F.  due 
to  the  resistance  of  the  intervening  liquid,  so  that  work 
is  expended  in  liberating  the  products  of  electrolysis. 
The  development  of  c.  E.  M.  F.  in  the  arc  lamp,  is  ana- 
logous to  the  development  of  c.  E.  M.  F.  by  electrolysis ; 
and,  in  point  of  fact,  the  voltaic  arc  with  its  carbon  elec- 
trodes, forms  a  species  of  electrolytic  cell,  the  carbon 
vapor  being  analogous  to  the  electrolyte.  The  energy  is 
here  expended  in  volatilizing  carbon  at  an  extremely 
high  temperature,  estimated  at  3,500°  C.;  in  fact  the 
temperature,  which  can  be  attained  by  means  of  the 
electric  arc,  is  probably  greater  than  can  be  obtained  in 
any  other  way.  The  c.  E.  M.  F.  of  an  arc  lamp  is  practi- 
cally the  same  for  the  same  distance  between  the  carbon 
points  for  all  dimensions  of  carbon  electrodes,  or  areas  of 
incandescence.  But  the  larger  the  carbon,  and  the 
greater  the  surface  of  incandescence,  the  greater  the  cur- 
rent strength  that  must  be  supplied  to  it.  For  very 
large  arcs,  such  as  in  powerful  search-lights,  a  current 
strength  of  as  much  as  200  amperes,  is  sometimes  em- 
ployed, requiring,  therefore,  an  activity  of  about  10  K.  w. 

234.  During  the  maintenance  of  the  arc,  the  positive 
carbon,    that    is,   the    carbon    from   which    the 

current  enters  the  arc,   attains    at  its   crater,  a   much 


219 


higher  temperature  than  that  of  the  incandescent  carbon. 
Indeed,  the  temperature  of  the  negative  carbon  is  suffici- 
ently lower  to  permit  the  condensation  of  carbon  vapor 
on  its  surface,  so  that  after  the  arc  has  been  maintained 
for  some  time,  a  nipple  will  be  formed  on  the  end  of  the 
negative  carbon  opposite  the  crater  in  the  positive.  The 
material  so  deposited  is  pure  graphite.  During  the 
maintenance  of  the  arc,  the  carbon  vapor  being  exposed 
to  the  air,  is  consumed  by  oxidation.  The  rate  of  con- 
sumption of  the  positive  carbon,  however,  is  greater  than 
that  of  the  negative,  owing  to  the  fact  that  it  is  vola- 
tilized. Roughly  speaking,  the  rate  of  consumption  of 
the  positive  carbon  is  twice  that  of  the  negative. 

235.  In  the  early  history  of  arc  lighting,  the  carbon 
electrodes  employed  were  sawn  out  of  blocks  of 
the  hard  deposits  of  carbon  found  inside  the  gas"  re- 
torts, employed  in  the  manufacture  of  illuminating  gas 
by  the  destructive  distillation  of  coal.  The  enormous 
demand  for  carbon  electrodes,  however,  soon  rendered 
this  source  insufficient,  and  now,  all  carbon  electrodes 
are  manufactured.  The  process  is  essentially  as  follows  : 
Pulverized  carbonaceous  substances,  such  as  powdered 
coke  or  charcoal,  are  mixed  into  a  stiff  paste  with  some 
carbonaceous  liquid,  such  as  coal-tar,  and  are  then  moulded 
or  forced  through  a  die  under  great  hydraulic  pressure, 
dried,  and  submitted  to  a  carbonizing  process  by  baking 
in  a  furnace.  During  this  process  the  cohesion  of  the 
carbon  powders  is  increased  by  the  carbon  deposited  from 
the  decomposition  of  the  carbonizable  liquid  under  the 
influence  of  the  heat.  Since,  in  nearly  all  the  arc  lights 
in  practical  use,  the  carbons  are  placed  vertically  one 
above  the  other,  it  is  necessary  to  make  the  carbon  rods 


220 


or  pencils  very  nearly  straight,  so  that  their  axes  may 
coincide  during  feeding.  Where  an  exceedingly  hard 
variety  of  carbon  is  required,  the  expedient  is  some- 
times adopted  of  soaking  the  carbons,  after  carbonization, 
in  some  carbonaceous  liquid,  and  again  subjecting  them 
to  a  further  process  of  carbonization,  but  this  is  only 
adopted  in  the  manufacture  of  carbons  for  special  pur- 
poses. 

236.  The  steadiness  of  the  arc  light,  though  dependent 
on  a  variety  of  circumstances,  is  largely  influenced 

by  the  position  occupied  by  the  arc.  In  order  to  prevent 
a  travelling  of  the  arc  around  different  portions  of  the 
edge  of  the  carbon,  the  expedient  is  sometimes  adopted 
of  making  the  central  portions  of  the  electrodes  softer, 
that  is,  more  readily  volatilized,  than,  the  remaining  ma- 
terial, by  the  introduction  of  a  different  kind  of  carbon. 
Such  carbons  are  called  cored  carbons.  Owing  to  their 
expense,  they  are  not  extensively  used  in  commercial 
lighting. 

The  carbon  in  general  use,  is  the  ordinary  coreless  car- 
bon which  has  been  electrolytically  coated  with  a  deposit 
of  metallic  copper.  Although  uncoated  carbons  are  fre- 
quently employed,  yet,  unless  special  care  is  taken  in 
their  manufacture,  they  are  apt  to  burn  irregularly  on 
the  sides,  and  becoming  pointed,  are  apt  to  interfere 
with  the  proper  operation  of  the  lamp. 

237.  The  candle-power  of  an  arc-lamp  is  very  differ- 
ent in  different  directions,  and,  since  in  practice,  the 

arc  rarely  remains  for  any  length  of  time  in  a  fixed  posi- 
tion between  the  carbons,  the  candle-power  as  indicated  by 
a  photometer,  is  constantly  varying.  Fig.  92  represents 


221 


diagram  matically  |  the  physiologically  effective  luminous 
intensity  of  an  ordinary  arc  lamp,  at  different  angular 
positions  ahout  the  carbons  as  an  axis.  It  will  be  observed 
that  at  an  angle  of  about  50°  below  the  horizontal  plane, 
when  the  carbons  are  vertical,  the  intensity  is  a  maxi- 
mum, and  that  it  rapidly  diminishes  both  above  and  be- 
low this  position.  The  mean  spherical  candle-power  is  the 
average  candle-power  taken  all  over  the  surface  of  a 
sphere  having  the  arc  at  its  centre,  and  is  usually  about 
one  third  of  the  maximum  candle-power,  and  capable  of 


Elec.  Engineer 

FIG.  92. 

Diagram  Indicating  Luminous  Intensity  of  an  Arc  Lamp  in  Different  Directions. 

being  expressed  with  a  fair  approach  to  accuracy  by  the 
numerical  formula 
Mean  spherical  candle-power  = 
Mean  horizontal  candle-power     maximum  candle  power 

~2~~  ~T~ 

The  existing  practice  of  rating  the  luminous  power  of 
an  arc  lamp  by  its  maximum  luminous  intensity,  is  very 
defective,  and  could  be  preferably  replaced  by  a  state- 
ment of  the  mean  spherical  candle-power,  or  the  total  flux 
of  light,  (physiologically  effective  radiation). 


222 


It  may  be  supposed  that  the  horizontal  candle-power 
of  a  lamp,  that  is,  its  candle-power  in  the  horizontal 
plane,  would  be  the  same  in  all  directions.  This,  how- 
ever, is  not  the  case,  owing  to  the  fact  that  the  carbons 
burn  irregularly,  and  that  the  crater,  which  forms  the  main 
source  of  light  of  the  voltaic  arc,  is  seldom  either  located 
exactly  centrally  at  the  end  of  the  positive  carbon,  or  is 
surrounded  by  walls  of  equal  height.  It  becomes  neces- 
sary, therefore,  to  take  the  mean  horizontal  candle-power 
of  a  lamp,  which  is  usually  only  a  small  fraction  of  its 
maximum  candle-power. 

238.  Since  the  carbons  are  consumed  during  use,  and 
the  steadiness  of  the  light  produced  requires, 
among  other  things,  that  the  length  of  the  arc  be  main- 
tained a  constant  distance  apart,  it  is  necessary  that  some 
regulating  device  be  employed,  whereby  the  carbons  can 
be  maintained  at  this  distance  during  use.  This  is  ac- 
complished by  means  of  various  feeding  mechanisms 
connected  with  the  lamp.  A  great  variety  of  feeding 
mechanisms  have  been  devised  depending  upon  the  posi- 
tion of  the  carbon  and  upon  whether  both  carbons  are 
fed  toward  each  other,  or  whether,  as  is  generally  the 
case,  the  negative  carbon  is  fixed  and  the  positive  or 
upper  carbon  alone  is  fed. 

The  carbons  have  been  placed  in  various  positions ; 
parallel  or  oblique,  that  is,  included  at  all  angles 
from  zero  to  180°.  At  one  time  in  the  history  of 
the  art,  the  carbons  were  employed  parallel,  as  in  the 
Jablochkoff  candles,  the  carbons  being  maintained  at 
a  constant  distance  apart,  not  by  the  use  of  the  feeding 
mechanism  but  by  means  of  non-conducting  substances 
such  as  kaolin  placed  between  and  consumed  with  them. 


223 


Nearly  all  commercial  systems  of  arc-lighting  at  the 
present  day,  employ  the  carbons  vertically  over  one 
another,  with  the  positive  carbon  uppermost,  except 
where  the  walls  of  buildings  are  to  be  illumined  from 
lamps  placed  beneath,  when  the  negative  carbon  may  be 
placed  beneath.  This,  of  course,  is  on  account  of  the 
fact  that  the  crater  in  the  positive  carbon  is  the  main 
source  of  light,  the  greater  intensity  of  light  being  pro- 
jected directly  from  the  crater,  which,  when  the  positive 
is  the  upper  carbon,  will  be  downwards  as  will  be  seen 
by  an  inspection  of  the  carve  in  Fig.  92. 

239.  For  most  commercial  purposes  a  slight  change 
in  the  height  of  the  arc  within  the  lamp  globe 

is  immaterial.  Consequently  the  use  of  mechanism,  feed- 
ing one  of  the  carbons  only  is  not  objectionable,  provided 
the  length  of  the  negative  carbon  is  so  adjusted  that  the 
position  of  the  arc  shall  never  fall  outside  the  surrounding 
glass  globe.  For  light-house  purposes,  and  for  search- 
lights generally,  where  the  arc  is  used  in  connection  with 
reflectors  and  the  position  of  the  arc  is  therefore  impor- 
tant, the  mechanism  of  the  lamp  is  adapted  to  feed  both 
carbons.  In  such  cases  the  negative  carbon  is  fed 
about  one  half  as  rapidly  as  the  positive  carbon. 

240.  The  result  of  experience  has  been  to  limit  the 
length  of  the  carbon  used  for  the  positive  elec- 
trode to  about  12  inches.     For  long  runs  during  winter, 
varying  from,  say,  13  to  15  hours,  a  single  pair  of  car- 
bon pencils,  would  be  insufficient  and  would,  therefore, 
necessitate  recarboning  during  the  night.     In  order  to 
avoid  this,  the  device  of  employing  two  separate  pairs  of 
carbons  has   been   adopted,  the  circuit  connections  be- 


224 


ing  such  that  when,  by  consumption,  the  length  of  one 
pair  of  carbons  has  been  reduced  a  certain  predetermined 
amount,  the  current  is  automatically  trans- 
ferred to  the  second  pair.  A  doiible-carbon, 
or  all-night  arc  lamp  of  this  character  is 
shown  in  Fig.  93. 

SYLLABUS. 

In  the  carbon  voltaic  arc,  an  activity  of 
about  450  watts  is  usually  expended,  with  about 
10  amperes  at  45  volts  pressure. 

The  c.  E.  M.  F.  in  the  circuit  is  principally 
developed  at  the  surface  of  the  positive  carbon 
or  crater  where  work  is  being  done  in  volati- 
lizing carbon  against  its  cohesive  attraction. 
During  the  maintenance  of  the  arc,  car- 
bon is  volatilized  mainly  from  the  end  of 
the   positive  carbon,  some  of  the  volatilized 
Double  carbon  carbon  being  deposited  as  a  nipple  of  graphite, 
Arc  Lamp.    on  ^g  Qppogijjg  surface  of  the  negative  carbon. 
The  mean   spherical  candle-power  of  an  arc  light  is 
usually  half  the  mean  horizontal  candle-power,  plus  one 
quarter  of  the  maximum  candle-power. 

Laboratory  of  Houston  &  Kennelly, 
Philadelphia. 


Copyright,  1894,  by  THE  ELECTRICAL  ENGINEER.] 


WEEKLY. 


Price»    '    10  Cents- 
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Electrical   Engineering  Leaflets, 


Prof.  E.  J.  Houston,  Ph.  D. 

AND 

A.  E.  Kennelly,  F.  R.  A.  S. 


INTERMEDIATE   CRADE 

ARC  LIOHTINQ. 


24:1.     In     arc  lamps   when   the    current   is   off,   the 

feeding  mechanism  permits  the  upper  carbon  to 

fall,  until  it  rests  on  the   top   of  the  lower  carbon,  so 

that   the   carbons   are  in  contact  when    the   current  is 

started. 

On  the  passage  of  the  current  through  the  lamp,  an 
electromagnet  in  the  main  circuit,  by  the  attraction  of 
its  armature,  separates  the  positive  carbon  the  proper 
distance  from  the  negative,  thus  establishing  an  arc  be- 
tween them,  and  holds  the  upper  carbon  in  this  position 
by  the  operation  of  a  device  called  a  clutch.  When  the 
consumption  of  the  carbons  increases  the  distance  between 
them,  the  pressure  rises  at  the  terminals  of  the  lamp,  due 
to  the  extra  drop  in  the  increased  resistance  and  length 
of  the  arc,  and  as  soon  as  the  pressure  at  the  lamp  ter- 
minals has  risen  sufficiently  high,  i.  e.,  when  the  carbons 
have  burnt  a  certain  distance  apart,  a  special  magnet, 
wound  with  fine  wire,  so  that  its  resistance  is,  say  500 

Published  by 

THE   ELECTRICAL  ENGINEER, 
203  Broadway,  New  York,  N.  Y. 

[Entered  as  second-class  matter  at  the  New  York,  N.  Y.,  Post  Office,  June  14,  1894.] 


22G 


ohms,  placed  in  shunt  to  tlie  lamps,  attracts  its  armature, 
releases  the  clutch  and  permits  the  upper  carbon  to  fall. 
In  a  well  regulated  lamp,  the  upper  carbon,  while  in 
use,  never  actually  touches  the  lower  carbon,  since  the 
decrease  in  potential,  caused  by  the  decrease  in  the  re- 
sistance of  the  arc,  reduces  the  attraction  of  the  shunt 
magnet,  thus  allowing  the  clamp  to  clutch  the  upper 
carbon  swiftly. 

242.     In  all  series-connected  arc  lamps,  some  device 

must  be  employed  to  prevent  the  failure  of  any 

lamp  from  opening  the  entire  circuit.     This  is  generally 


Etec.  Engineer 

FIG.  94. 

Diagram  of  a  Form  of  Arc  Mechanism. 

accomplished  by  means  of  a  special  cut-out  magnet? 
placed  in  the  shunt  circuit,  and  so  arranged,  that,  whenever 
the  pressure  at  the  lamp  terminals  rises  beyond  a  certain 
working  maximum,  this  magnet  shall  operate  and  cut- 
out the  lamp,  by  releasing  a  spring,  short-circuiting  the 
lamp  terminals. 

The  connections  of  such  an  automatic  cut-out  are  di- 
agram matically  represented  in  Fig.  9±,  as  applied  to  a 
form  of  feeding  mechanism  for  a  series-connected  lamp. 
Here  the  lifting  magnet  M,  wound  with  coarse  wire  and 
having  a  resistance  of  about  0.05  ohm,  is  connected  di- 


22' 


rectly  in  circuit  with  the  arc.  On  the  attraction  of  the 
armature,  which  is  pivoted  at  B,  the  clutch  grips  the  lamp 
rod,  and  thus  raises  the  upper,  or  positive  carbon,  and 
establishes  an  arc.  When,  by  the  consumption  of  the 
carbons,  the  arc  becomes  too  long,  the  pressure  between 
the  carbon  electrodes  increases  and  more  current  flows 
through  the  magnet  N,  of  about  400  ohms  resistance,  wound 
with  fine  wire  and  placed  in  a  shunt  circuit  around  the 
electrodes.  As  soon  as  this  current  reaches  a  certain  criti- 
cal strength,  the  attraction  on  the  armature  momentarily 
releases  the  clutch  and  permits  the  upper  carbon  to  fall, 
until  by  decrease  in  the  length  of  the  arc,  the  current 
through  the  shunt  magnet  decreases,  when  the  upper  mag- 
net again  overpowers  it  arid  reclutches  the  upper  carbon. 

The  automatic  cut-out  mechanism  is  shown  at  s.  It 
consists  of  an  electro-magnet,  wound  witii  fine  wire^  and 
placed  in  shunt  around  the  carbon  electrodes.  If  the 
carbon  for  any  reason  fails  to  feed,  the  increased  pres- 
sure on  the  terminals  of  the  shunt  circuit  causes  the  mag- 
net s,  to  be  so  highly  energized  as  to  attract  its  armature 
and  thereby  permit  a  spring  automatically  to  close  the 
short-circuit  at  A,  and  cut  the  lamp  out. 

Besides  the  mechanism  described,  a  great  variety  of 
other  forms  have  been  designed  for  operating  arc  lamps. 
The  two  windings  N  and  M,  may  be  placed  on  the  same 
core  ;  or  they  may  be  placed  on  separate  magnets  attract- 
ing separate  armatures,  but  in  all  cases  a  series-winding  is 
employed  for  the  lifting  magnet,  and  a  shunt  winding  for 
the  feeding  magnet. 

243.     The  number  of   arc  lights  connected  in  series 

in  a  single  circuit  may  sometimes  be  as  great  as 

200,  representing  an  aggregate  pressure  at  the  generator 


228 


terminals  of  roughly  10,000  volts  (10  kilovolts).  More 
usually,  however,  125  lights  is  the  limit,  and  in  ordinary 
practice,  from  50  to  65.  For  street  lighting  purposes, 
from  9  to  10  amperes  is  the  strength  of  current  main- 
tained. Taking  the  average  number  of  arc  lights  on  a 
single  circuit  at,  say  60,  representing  an  aggregate  pres- 
sure of  3000  volts,  such  a  system  readily  adapts  itself  to 
lighting  an  extended  area,  since  the  size  of  wire  employed, 
usually  No.  6.  A.W.G.,  has  a  resistance  of  only  about  2.1 
ohms  per  mile.  Thus  a  circuit  of,  say,  live  miles  in 
length  would  only  have  a  resistance  of  10.5  ohms  in  con- 
ductors, producing  a  drop  of  105  volts,  with  10  amperes 
of  current,  which  would  represent  an  activity  of  1050 
watts,  and  would  be  capable  of  supplying  about  2  arc 
lamps. 

The  price  asked  for  arc  lighting  service  per  year  will, 
of  course,  depend  upon  a  variety  of  circumstances,  such 
as  the  size  and  nature  of  the  plant,  the  cost  of  coal  or 
water  power,  and  the  area  of  lighting,  etc.;  but  taking 
the  average  case,  the  price  would  probably  be  from  $70 
to  $110  per  450-watt,  50-yolt  arc  lamp  per  annum. 

244.  Arc  lamps  are  frequently  operated  on  incandes- 
cent circuits,  usually  two  in  series  on  110  volt 
circuits,  or  four  in  series  across  the  outside  conductors  of 
three  wire  circuits  (220  volts).  In  all  constant-potential 
lamps,  it  is  necessary  to  insert  a  resistance  in  the  circuit 
of  the  lamp  so  as  to  ensure  their  proper  operation.  This  is 
not  necessary  in  series-connected  lamps,  since  the  lamps 
tend  to  automatically  check  one  anothers  variations.  For 
this  reason  the  pressure  at  the  terminals  of  a  constant- 
potential  arc  lamp  will,  by  reason  of  the  drop  in  the  re- 
sistance, be  two  or  three  volts  greater,  than  in  the  case  of 


229 


series  lamps.  It  would  appear,  therefore,  that  the  effi- 
ciency of  a  series  arc  system  would  necessarily  be  greater 
than  that  of  the  same  number  of  lamps  operated  on 
constant-potential  circuits.  This,  however,  is  not  always 
the  case,  owing  partly  to  the  fact  that  a  series  generator 
has  a  somewhat  lower  efficiency  than  a  constant-potential 
generator.  Moreover,  when  a  constant-potential  incan- 
descent circuit  already  exists,  and  but  comparatively  few 
arc  lamps  are  required,  it  may  be  more  economical  to 
connect  these  directly  to  the  incandescent  circuit  than  to 


Q£) 

I 
1 

D.P  Snap  Switch 

i 
i 

ELEC.ENGH.  NV. 

^ 

FIG.  95. 

Incandescent  Circuit,  with  Standard  Lamps. 

install  a  separate  generator  and  circuit  for  their  special  ac- 
commodation. Fig.  95  shows  the  connections  of  a  pair 
of  arc  lamps  operated  in  series  from  a  pair  of  incandescent 
mains  at  about  110  volts  pressure. 

245.  In  a  station  for  the  operation  of  an  extensive 
system  of  arc  lamps,  where  a  number  of  dynamos 
are  employed  in  supplying  the  different  circuits,  and 
where  the  load  on  such  circuits  may  vary,  a  switchboard 
becomes  necessary,  whereby  this  load  can  be  readily  shifted 
from  one  dynamo  to  another.  Such  a  switch-board  re- 


230 


quires  a  high  insulation  on  account  of  the  pressures  em- 
ployed and  means  must  be  adopted  to  prevent  an  arc  being 
accidentally  drawn  from  one  bar  to  another.  Fig.  96 
represents  a  form  of  such  switchboard,  in  which  the 
connections  are  established  by  flexible  cords  connected 
with  suitably  insulated  handles  and  protected  by  rubber 
tubes. 


FIG.  96. 

Form  of  Series  Arc  Switchboard. 

24-6.  Arc  lamps  are  sometimes  operated  on  alterna- 
nating  current  circuits.  In  such  cases,  since  the 
direction  of  the  current  rapidly  changes,  a  definite  posi- 
tive crater  and  its  opposing  negative  nipple  are  never 
formed.  Consequently,  the  temperature  of  the  two 
carbons  is  approximately  the  same,  as  also  the  amount  of 
light  they  emit.  For  the  same  reason  the  distribution  of 


231 


light  is  more  regular,  than  in  the  case  of  the  continuous 
current  arc,  and  possesses  two  points  of  maximum  inten- 
sity, one  directed  upwards  and  one  downwards,  as  is 
shown  in  Fig.  97.  The  mean  horizontal  intensity  also 
bears  a  greater  proportion  to  the  mean  spherical  intensity, 
or  to  the  maximum  intensity  than  in  the  case  of  the  con- 
tinuous current  arc,  but  this  proportion  is  more  variable 
in  alternating  than  in  continuous  current  arcs. 

A  B 


KORIZQ 


FIG.  97. 

Distribution  of  Light  from  an  Alternating  Current  Arc  as  measured  in  a  particular  case. 

Arc  lamps  on  alternating  current  circuits  require  from 
28  to  35  volts  at  their  terminals,  according  to  the  charac- 
ter, size  and  separation  of  the  carbons.  All  alternating 
arcs  are  apt  to  produce  a  humming  sound,  the  pitch  of 
which  depends  upon  the  frequency  of  alternation.  This 
is  due  to  the  periodic  expansion  and  contraction  of  the 
air  in  the  successive  waves  of  heat  produced.  Alterna- 
ting-current arc  lamps  are  usually  supplied  by  transfor- 
mers, whose  primaries  are  connected  in  series  in  the 
main  circuit  and  whose  secondaries  are  locally  connected 

o*/  mn 

[TJNI7BESIT7; 


232 


to  each  arc  lamp.  The  current  employed  in  the  primary 
circuit,  instead  of  being  from  9  to  10  amperes,  the  usual 
strength  for  continuous  series-connected  circuits,  is  com- 
monly about  30  amperes,  the  secondary  current  strength 
in  the  lamp  circuits  being,  however,  about  9  amperes. 
The  consumption  of  the  carbons  is  nearly  uniform  in  an 
alternating  current  arc  lamp. 

SYLLABUS. 

Most  arc  lamp  mechanisms  in  use,  consist  of  an  elec- 
tromagnet placed  in  the  main  circuit  for  causing  a  sepa- 
ration of  the  carbons,  and  another  electromagnet,  placed 
in  a  shunt  circuit,  for  causing  their  approach.  In  all  se- 
ries-connected arc  lamps,  an  automatic  cut-out  device, 
operated  by  a  shunt  magnet,  is  provided  for  closing  a 
short  circuit  past  the  lamp  on  the  failure  of  its  carbons  to 
feed. 

Arc  lamps  are  sometimes  connected  in  a  single  series 
circuit  up  to  the  number  of  200.  More  commonly  125  is 
the  limiting  number,  while  from  50  to  65  is  the  number 
commonly  employed. 

Under  certain  circumstances  it  is  more  economical  to 
connect  arc-lamps  to  constant-potential  mains.  Such 
lamps  require  a  special  resistance  introduced  into  their 
circuit  in  order  to  control  them. 

Alternating  current  arcs  provide  a  more  general  dis- 
tribution of  light,  than  constant  current  arcs,  owing  to 
the  fact  that  both  carbons  are  at  approximately  the  same 
temperature. 

Laboratory  of  Houston  &  Kennelly, 
Philadelphia. 


[Copyright,  1894,  by  THE  ELECTRICAL  ENGINEER.] 
WEEKLY. 

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Electrical    Engineering   Leaflets, 


Prof.  E.  J.  Houston,  Ph.  D. 

AND 

A.  E.  Kennelly,  F.  R.  A.  S. 


INTERMEDIATE    GRADE. 

Alternating  Currents. 


247.  A  continuous  E.  M.  F.  or  current  not  only  con- 
tinuously preserves  the  same  direction,  but,  un- 
less otherwise  specified,  maintains  the  same  strengthr~An 
alternating  E.  M.  F.  or  current  is  one  which  changes  its 
direction,  being  alternately  positive  and  negative.  A  con- 
tinuous current  may  become  fluctuating  or  pulsatory :  i.e., 
it  may,  while  preserving  the  same  direction  of  current 
flow,  vary  either  periodically  or  irregularly  in  its  strength, 
but  an  E.  M.  F.  or  current  does  not  become  alternating 
unless  it  actually  changes  its  direction. 

The  difference  between  an  alternating  and  a  fluctuat- 
ing or  pulsatory  current  will  be  seen  from  an  inspection 
of  Fig.  98,  where  a  fluctuating  E,  M.  F.  or  current, 
although  represented  as  periodically  varying  in  intensity, 
is  not  alternating  since  it  is  constantly  directed  or  flows 
in  the  same  direction  through  the  conductor,  being  at 
all  times  represented  by  a  line  above  the  zero  line  o  A  ; 
while  an  alternating  E.  M.  F.  or  current  is  represented  by 

Published  by 

THE  ELECTRICAL  ENGINEER, 
203  Broadway,  New  York,  N.  Y. 

[Entered  as  second-class  matter  at  the  New  York,  N.  Y.,  Post  Office,  June  14,  1894.] 


234 


a  line  which  is  alternately  on  the  positive  and  negative 
sides  of  the  zero  line,  o  A. 

The  term  alternating  E.  M.  F.  or  current,  a?  employed 


Elec.bnginter 

FIG.  98. 

Fluctuating  and  Alternating  E.  M.  F.'S  or  Currents. 

in  practice,  conveys  the  conception  not  only  of  periodic 
alternation  of  direction,  but  also  of  periodic  recurrence  of 
magnitude.  In  other  words,  if  an  alternating  current  or 
E.  M.  F.  be  graphically  represented  by  a  curve,  whatever 
may  be  the  shape  of  this  curve  as  representing  direction 
and  magnitude,  this  shape  must  be  repeated  in  successive 
waves. 

There  may  be  an  infinite  variety  of  alternating  E.  M.  F.'S 
and  currents,  not  simply  in  regard  to  their  magnitude, 
but  also  in  regard  to  their  manner  of  variation,  as  shown 
in  Fig.  99. 


c 

/ 

j 

.  IP 

1 

8 

= 

c 

I         ' 

k 

FIG.  99 

Periodic  Alternating  E.  M.  F.  or  Current. 
Rectangular  Type. 


Elec.  Engineer 

FIG.  100. 


Periodic  Alternating  E.  M.  F.  or  Current. 
Zig-zag  Type. 


The  E.  M,  F.  may  suddenly  reverse  its  direction,  as,  for 
example,  by  the  action  of  a  commutator,  so  that  the 
E.  M.  F.  may  suddenly  change  from  a  positive  maximum 


235 


to  a  negative  maximum,  and  vice  versa,  of  which  the 
graphical  representation  is  the  flat-topped  type  of  wave ; 
or,  the  E.  M.  F.  may  gradually  increase  and  decrease 
at  a  uniform  rate  from  the  positive  to  the  negative 
maxima,  and  vice  versa  as  shown  in  Fig.  100,  whose 
graphical  representation  is  a  wave  of  the  zig-zag  type. 
E.  M.  F.'S  or  currents  of  this  type  seldom  exist  in  practice, 
but  approximations  to  them  exist,  of  the  types  shown  in 
Fig.  101,  which  represents  a  type  of  alternating  wave  of 
the  flat-topped  variety,  and  in  Fig.  102,  which  represents 
a  type  of  the  peaked  variety  of  wave,  such  as  some  alter- 
nators produce. 


FIG.  101. 


FIG.  102. 


ec.  Engineer 

FIG.  103. 


Periodic  Alternating  E.M.F.      Periodic  Alternating  E.M.F.      Periodic  Alternating  E.M.F  • 

or  Current.  or  Current.  or  Current. 

Flat  Topped  Curve.  Peaked  Curve.  Sinusoidal  Curve. 

The  E.  M.  F.  may  assume  a  wave  form  intermediate  be- 
tween the  flat-top  and  the  peaked  varieties,  and  called 
the  sinusoidal  form,  shown  in  Fig.  103,  because  its 
graphical  representation  is  a  sinusoid  or  curve  of  sines. 

The  sinusoidal  form  of  wave  may  be  understood  from 
a  consideration  of  the  following  preliminary  principles ; 
namely,  if  the  disc  Q  R  s,  Fig.  104,  supported  on  a  hori- 
zontal axis  A  B,  at  o,  be  uniformly  rotated  about  this  axis, 
the  vertically  falling  shadow  ^>,  of  the  point  p,  situated  on 
the  radius  o  P,  intercepted  by  a  horizontal  sheet  of  paper 
E  F  G  H,  will  execute  a  to-and-fro  motion  along  the  line, 
p  o  q,  whose  length  will  be  twice  the  radius  o  P,  and  the 


236 


shadow  will  occupy  different  positions  on  this  path  accord- 
ing to  the  different  positions  of  the  disc.  The  motion  of 
the  shadow  thus  produced  is  called  a  simple-harmonic  or 
simple-periodic  motion  and  the  E.  M.  F.  or  current, 
whose  magnitude  varies  in  accordance  with  such  motion 
is  called  a  simple-harmonic  or  simple-periodic  E.  M.  F.  or 
current.  If  now,  the  sheet  of  paper  be  moved  steadily 
forwards  in  the  horizontal  plane,  parallel  to  the  axis  A  B, 
the  moving  shadow  will  trace  on  its  surface  a  wave 
curve  of  the  type  shown  in  Fig.  103,  and  called  a  sinu- 
soid, because  the  distance  of  any  point  on  the  curve 


Elec.  Engineer 


Fro.  104. 

Diagram  of  Simple  Harmonic  Motion. 

from  the  zero  line  a  &,  measures  the  sine  of  the  angle 
at  that  moment  included  between  the  radius  vector  o  P, 
and  the  vertical  plane. 

The  shape  of  the  sinusoid  will  depend  upon  the  length 
of  the  radius  vector  and  on  the  speed  with  which  the 
disc  rotates,  as  shown  in  Fig.  105.  For  example,  if  the 
radius  vector  have  the  value  o  j,  as  shown  at  A,  the  sinu- 
soid traced  for  a  particular  speed  of  disc  and  paper  is 
shown  by  the  curve  A  B  c  D  E  F.  If  now,  the  velocities  re- 
maining the  same,  the  radius  vector  be  halved,  as  at  B, 


23' 


the  resulting  sinusoid  will  be  flattened.  On  the  con- 
trary, if,  as  at  D,  the  radius  vector  remain  as  before,  but 
the  velocity  of  rotation  be  doubled,  the  sinusoid  will 
be  sharpened. 

When  a  conducting  loop  or  coil  is  steadily  rotated 
about  any  diameter  in  a  uniform  magnetic  flux,  a  sinu- 
soidal or  simple-periodic  E.  M.  F.  will  be  generated  in  it. 


JHec+Engineer 

FIG.  105. 

Graphical  Representations  of  Simple  Harmonic  E.  M.  F.'.  or  Currents. 

Commercial  alternators,  i.  e.,  alternating  current  gen- 
erators, do  not  produce  true  sinusoidal  E.  M.  F.'S,  but  in 
many  instances  they  produce  so  close  an  approximation 
thereto  that,  for  the  purposes  of  computation,  their  val- 
ues may  be  regarded  as  sinusoidal.  Even  when  the 
wave  of  E.  M.  F.  generated  by  an  alternator  is  distinctly 
flat-topped  or  peaked,  its  E.  M.  F.  is  usually  regarded  as 


238 


sinusoidal  to  a  first  approximation,  and  corrections  are 
subsequently  introduced  for  the  effects  of  deviation. 

An  alternation,  or  semi-period,  consists  of  a  single 
wave  in  either  the  positive  or  negative  direction.  A  com- 
plete double  alternation,  that  is,  a  double  reversal,  consti- 
tutes a  cycle.  Thus  the  wave  o  a  ft  <?,  or  c  d  e  f,  Fig. 
101,  represents  a  reversal  or  alternation,  but  the  double 
reversal  o  a  b  c  d  ef,  or  c  d  efg  fij,  constitutes  a  cycle, 
since  the  generating  point  returns  at  the  end  to  the  ini- 
tial position.  A  cycle  need  not  necessarily  commence 
and  terminate  at  the  zero  point ;  thus  b  c  defgh,  would 
constitute  a  cycle. 

The  time  occupied  by  an  alternating  E.  M.  F.  or  current 
in  completing  a  cycle  is  called  a  period.  The  period 
employed  in  commercial  alternating  current  apparatus 
varies  from  about  0.008  to  0.04  second.  In  any  alter- 
nating current  circuit,  the  period  of  the  current  must  al- 
ways be  equal  to  the  period  of  the  E.  M.  F.  in  the  circuit. 

The  number  of  periods  in  a  second  is  called  t\\e  fre- 
quency. Thus  the  frequency  in  commercial  alternating 
current  apparatus  varies  between  25  ~,  that  is,  25  cycles 
per  second  and  133  ~.  The  frequency  is  sometimes  ex- 
pressed by  the  number  of  alternations  per  minute. 
Thus  an  alternator  may  be  described  as  producing  16,000 
alternations  per  minute.  This  corresponds  to  8000  ~  or 
periods  per  minute  and,  therefore,  to  133.3  ~  per  second. 

Assuming  an  alternating  E.  M.  F.  or  current  to  be  sinu- 
soidal, its  phase  is  the  angle  between  the  imaginary  radius 
vector  and  the  initial  descending  radius  where  the  tracing 
point  starts  from  the  zero  line  in  the  positive  direction. 
Thus  the  phase  at  D,  Fig.  105  A,  is  zero,  at  E  or  A,  is  90°, 
at  B  or  F,  is  180°  and  at  c,  270°. 


239 


248.  When  we  describe  the  magnitude  of  a  continu- 
ous E.  M.  F.  or  current  we  simply  state  its  constant 
strength,  but,  since  the  strength  of  an  alternating  cur- 
rent is  constantly  varying,  some  convention  is  necessary 
in  order  adequately  to  describe  its  magnitude.  The 
maximum  magnitude,  attained  during  each  cycle,  that 
is,  its  amplitude,  is  not  sufficient,  owing  to  the  very  dif- 
ferent shapes  of  different  types  of  wave  ;  nor  is  the  mean 
or  average  value  of  the  current,  taken  without  regard  to 
direction,  a  sufficient  criterion  for  most  practical  pur- 
poses. The  heating  effect  of  electric  currents  or  E.  M.  F. 
being  their  most  important  characteristic  from  a  practical 
point  of  view,  the  strength  of  an  alternating  E.  M.  F.  or 
current  is  conventionally  defined  as  its  effective  or  equiv- 
alent heating  value  in  a  continuous  current  circuit.  Thus, 
if  a  continuous  current  of  one  ampere  is  capable  of  develop- 
ing a  thermal  activity  of  a  certain  number  of  watts,  in  a 
resistance  through  which  it  passes,  then  any  alternating 
current  passing  through  the  same  resistance,  which  pro- 
duces the  same  thermal  activity,  has  a  strength  of  one 
ampere.  Since  the  thermal  activity  of  a  continuous 
E.  M.  F.  or  current  varies  with  its  square,  the  instantane- 
ous thermal  activity  in  any  alternating  E.  M.  F.  or  current 
similarly  varies  with  its  square.  Thus,  if  the  curve 
A  B  c  D  E  F,  at  A9  Fig.  105,  represents  an  alternating  cur- 
rent of  which  the  amplitude  o  A,  is  one  ampere,  then  the 
rate  at  which  heat  will  be  developed  by  this  current  in  a 
resistance  of  one  ohm,  at  the  instant  of  time  correspond- 
ing to  the  ordinate  o  A,  will  be  I2  X  1=1  watt ;  at  the 
point  H,  where  the  current  strength  is  0.5  ampere,  the 
rate  of  developing  heat  in  the  resistance  will  be  (0.5)  2 
X  1  =  0.25  watt.  If,  proceeding  in  this  way,  we  were 


240 


to  average  the  rate  of  expending  heat  in  the  resistance 
coil  through  one  complete  cycle,  and  take  a  sufficient 
number  of  measurements  to  obtain  the  necessary  degree 
of  accuracy,  we  should  find  that  the  mean  rate  of  ex- 
pending heat  would  be  just  0.5  watt,  or  half  the  maxi- 
mum rate,  this  being  the  law  for  strictly  sinusoidal 
current  waves.  Since  the  strength  of  the  current 
must  be  such  as  will  develop  0.5  watt  thermally  in  one 
ohm,  its  effective  value  will  be  1/0.5  or  0.707  ampere. 
Consequently,  the  effective  value  of  a  sinusoidal  E.  M.  F. 
or  current  is  the  maximum  value  or  amplitude,  divided 
by  V2  ;  that  h,  multiplied  by  0.707. 

SYLLABUS. 

A  sinusoidal  E.  M.  F.  or  current  is  one  whose  graphical 
representation  is  a  sinusoidal  curve. 

The  effective  value  of  an  alternating  E.  M.  F.  or  cur- 
rent is  the  value  of  the  strength  of  continuous  E.  M.  F. 
or  current,  which  would  produce  the  same  thermal 
activity  ;  that  is,  which  would  produce  the  same  amount 
of  heat  in  the  same  (considerable)  amount  of  time. 

Laboratory  of  Houston  &  Kennelly, 
Philadelphia. 


[Copyright,  1894,  by  THE  KI.ECTRICAL  ENGINBER.] 


WEEKLY. 


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Electrical    Engineering   Leaflets, 


Prof.  E.  J.  Houston,  Ph.  D. 

AND 

A.  E.  Kennelly,  F.  R.  A.  S. 


INTERMEDIATE 

Alternating  Currents. 


240.  From  what  has  been  said  concerning  the  nature 
of  simple-harmonic  motion  generally,  it  is  evident 
that  in  sinusoidal  current  circuits  we  have  to  deal  with 
quantities  which  are  varying  with  time  according  to  the 
projection,  on  a  line,  of  circular  motion  in  a  plane,  as 
represented,  for  example,  in  Fig.  106.  It  is,  therefore, 
of  importance,  in  considering  such  circuits  to  bear  in 
mind  the  relation  of  geometrical  magnitudes,  as  opposed 
to  simple  arithmetical  magnitudes ;  that  is  to  say,  that 
not  only  the  magnitudes  of  the  various  E.  M.  F.'S  and  cur- 
rents have  to  be  considered,  but  also  their  directions  at 
different  instants  of  time.  For  example,  if  two  similar 
sinusoidal  alternators  be  rigidly  connected  on  the  same 
shaft,  and  coupled  in  series,  the  E.  M.  F.  of  each  will  have 
the  same  frequency  and  the  same  magnitude.  Their 
resultant  E.  M.  F.,  when  connected  in  series,  will  depend 
upon  the  exact  relative  position  of  the  two  armatures  to 
each  other  on  the  common  shaft ;  that  is,  upon  the  re- 

Published  by 

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[Entered  as  second-class  matter  at  the  New  York,  N.  Y.,  Post  Office,  June  14,  1894,] 


242 


lative  phase  of  the  E.  M.  F.'S  and  whether  these  are  in- 
step or  out-of-step  with  each  other.  If  they  are  exactly 
in-step,  that  is,  coupled  in  the  same  phase,  so  that  the 
two  waves  of  E.  M.  F.  are  synchronous,  or  rising  and  fall- 
ing exactly  together,  the  resultant  E.  M.  F.  of  the  com- 
bination will  be  the  simple  arithmetical  sum,  or  twice 
the  E.  M.  F.  of  either,  and  in-step  with  each,  so  that  if 
each  gives  separately  1000  volts  effective  E.  M.  F.  at  a 
frequency  of  120  ~,  the  two  so  connected  will  give  in 
series  a  total  E.  M.  F.  of  2000  volts  effective,  with,  of 
course,  the  same  frequency.  If,  however,  the  two  ma- 
chines are  coupled  in  exactly  opposite  phase,  so  that 


FIG.  106. 

while  one  develops  the  positive  crest  of  a  wave,  the  other 
develops  a  negative  crest,  so  that  the  difference  of  phase 
is  180°,  or  half  a  cycle  between  them,  then  their  E.  M.  F.'S 
at  all  moments  are  oppositely  directed,  and  the  total 
E.  M.  F.  of  the  combination  will  be  zero,  since  in  all  parts 
of  the  wave,  the  E.  M.  F.  of  the  one  will  exactly  neutral- 
ize that  of  the  other.  At  intermediate  positions  of 
coupling,  or  phase  difference,  the  resultant  E.  M.  F.  will 
vary  between  2000  volts  and  zero,  and  the  phase  of  the 
resultant  E.  M.  F.  will  also  vary. 

250.     This  resultant  E.  M.  F.  can  be  very  simply  deter- 
mined by   considering  each  E.  M.  F.  as  a  line,  or 
plane  vector,  revol  ving  about  one  extremity,  and  making 
as  many  revolutions  per  second  as  there  are  periods  per 
second  in  the  frequency.    Thus,  let  A  B,  Fig.  106,  repre- 


243 


sent  a  sinusoidal  E.  M.  F.  of  1180  volts  effective,  and  sup- 
pose that  this  line  revolves  counter-clockwise  in  the 
plane  of  the  paper,  about  the  centre  A,  120  times  in  a 
second.  At  the  moment  when  A  B,  is  in  the  position 
shown,  let  a  second  sinusoidal  E.  M.  F.  of  820  volts  effec- 
tive, CD,  of  the  same  frequency,  but  having  a  phase 
150°,  or  T5^  of  a  cycle  in  rear  of  A  B,  as  shown  by  the 
direction  of  the  line  c  D  ;  then  the  resultant,  or  sum  of 
these  two  sinusoidal  E.  M.  F.'S,  when  connected  in  series, 
is  represented  in  direction  and  magnitude  by  the  line 
A  D,  for  c  D  is  here  added  geometrically  to  A  B.  The 
line  A  D,  has  a  length  of  020  volts,  according  to  the 


FIG.  107. 

original  scale,  and  makes  an  angle  of  approximately  41° 
with  A  B,  so  that  the  resultant  E.  M.  F.  of  the  combina- 
tion will  be  620  volts  effective,  lagging  in  phase  41° 
behind  A  B,  or  advancing  in  phase  109°  beyond  c  D. 

251.  If  two  similar  sinusoidal  alternators  be  coupled 
in  series,  as  shown  in  Fig.  107,  so  that  one  E.  M.  F., 
F  G,  leads  the  other,  E  F,  by  60°,  or  -J  cycle,  then 
the  sum  of  these  two  E.  M.  F.'S  in  series  will  be  1733  volts 
effective.  30°  ahead  of  E  F.  Again,  if  they  be  connected 
in  quadrature,  that  is,  at  quarter  phase,  so  that  j  K, 
leads  H  .1  by  90°,  their  sum  in  series  will  be  1415  volts, 
45  c,  or  \  period,  beyond  H  j,  and  behind  j  K  ;  and  again, 


244: 


if  the  phase  difference  amounts  to  150°,  so  that  M  N, 
makes  30°  with  L  M,  their  sum  in  series  will  be  L  N  or 
518  volts  effective,  75°  in  advance  of  L  M. 

252.  The  current  strength  in  an  alternating  current 
circuit  is  not  that  which  would  be  immediately 

obtained  at  first  sight  from  Ohm's  law.  Ohm's  law  applies 
to  alternating  current  circuits  when  the  c.  E.  M.  F.'S  in  the 
circuit  are  considered;  but  without  taking  the  pains  to 
determine  what  the  c.  E.  M.  F.'S  become  in  an  alternating 
circuit,  we  may  consider  that  the  impressed  E.  M.  F.'S, 
that  is,  the  E.  M.  F.'S  produced  by  the  source  or  sources, 
are  alone  operative,  and  that  the  resistance  of  an  alter- 
nating-current circuit  is  different  from  that  of  the  same 
circuit  operated  by  continuous  currents ;  or,  in  other 
words,  that  the  resistance  becomes  converted  into  a 
hypothetical  quantity  called  the  impedance,  and  express- 
ible in  ohms.  Ohm's  law  applied  to  alternating  current 
circuits  is,  therefore, 

-p  Tfl 

I  =  ^  amperes,  instead  of  /  =  —  amperes, 
J  H 

where  J,  is  the  impedance. 

253.  There  are  two  quantities  which  combine  with 
resistance  to  make  up  the  apparent  resistance  or 

impedance  of  alternating-current  circuits,  namely  : 

(1)  Inductance,  as  typically  developed  in  choking  coils 
and  which  is  always  present  in  greater  or  less  degree ; 

(2)  Electrostatic   capacity,    as  typically  developed  in 
condensers,  and  which  in  some  circuits  is  almost  entirely 
absent,  but  at  other  times  exists  in  a  marked  degree. 

If  we  suppose  that  a  coil  of  insulated  copper  wire  has 
a   resistance  of  10  ohms  and  an  inductance,  i.e.,  a  self- 


245 


induction  of  0.015  henry,  and  that  a  sinusoidal  E.  M.  F. 
of  52  volts  at  a  frequency  of  100  ~  is  impressed  on 
this  coil,  then,  since  no  appreciable  electrostatic  capacity 
exists  in  the  circuit,  the  impedance  is  composed  of  two 
parts  ;  viz.,  of  the  resistance,  and  of  a  quantity  called  the 
rcacta/ncc,  due  to  the  inductance  of  the  coil.  This  re- 
actance is  determined  by  multiplying  the  inductance  by 
the  angular  velocity  of  the  E.  M.  F.  The  angular  velocity 
in  this  case  is  100  revolutions  per  second,  and,  since 
there  are  2  TT  radians  in  a  revolution,  200  TT,  or  628.3  rad- 
ians, per  second.  Multiplying  this  angular  velocity  by 
the  inductance  in  Henrys,  we  obtain  the  reactance : 
628.3  X  0.015  =  9.425  ohms.  The  reactance  is  always 
graphically  set  off.  at  right  angles  to  the  resistance  of  a 
circuit,  the  inductance-reactance  being  set  oif  above  the 
line.  Thus  a  J,  Fig.  108,  having  a  resistance  of  10  ohms, 
the  reactance  b  c,  9.425  ohms,  is  laid  off  above  the_line 
a  &,  and  the  impedance,  which  is  always  the  geometrical 
sum  of  the  resistance  and  reactance,  is  equal  to  the  length 
of  the  line  a  <?,  joining  a  and  c,  or  13.74  ohms.  If  we 
divide  this  impedance  into  the  pressure,  according  to  the 
modified  form  of  Ohm's  law  before  stated,  we  have 

52 
_  =  3.785  amperes,  the  effective  current  strength. 


13.74 

254.  The  reactance  of  a  condenser  is  equal  to  the  reci- 
procal of  the  product  of  the  angular  velocity  of  the 
E.  M.  F.  by  its  capacity  in  farads.  Thus,  if  a  10  microfarad 
condenser  be  connected  directly  across  the  terminals  of  an 
alternator,  supplying  1100  volts  effective,  at  a  frequency  of 
100  ~,  the  angular  velocity  will  be  628.3  radians  per  second 
as  before,  and  the  product  of  this  by  10  million ths  of  a 


246 


farad  will  be  (Fig.  10U) 


6283 


ciprocal  of  this  quantity  or 


1,000,000 

1 


=  0.006288.    The  re- 


=  159.2  ohms,  and 


0.0062*3 
the  current  passing  into  the  condenser  will   be,  by  the 

modified  form  of  Ohm's  law,  =    6.91    amperes 

159.2 
effective. 

255.     If  the  condenser  instead  of  being  connected  di- 
rectly across  the  terminals  of  the  alternator  were 
in  series  with  a  resistance  coil  of  50  ohms,  having  an  in- 

!L. 

5/1 


«-^4 


RESISTANCE  10  OHMS 


« 


FIG.  108. 


FIG.  109. 


ductance  of  0.02  henry,  then  it  is  necessary  to  determine 
the  impedance  of  a  circuit  composed  of  resistance,  in- 
ductance and  capacity  combined.  The  reactance  in  this 
case  will  be  partly  due  to  inductance  and  partly  due  to 
capacity.  The  inductance-reactance  will  be  0.02  X  628.3 
—  125.7  ohms,  and  the  capacity- reactance  will  be,  as  be- 
fore, 159.2  ohms;  but,  while  inductance-reactance  is  al- 
ways laid  off  above  the  resistance  line,  capacity-reactance 
is  always  laid  off  below  the  resistance  line,  or  in  the  oppo- 
site direction,  because  capacity  and  inductance  tend  to  neu- 
tralize each  other's  influence.  The  resultant  reactance 
in  this  case,  as  shown  in  Fig.  110,  or  33.5  ohms,  will, 


247 


therefore,  be  directed  downwards,  and  the  impedance  of 
the  circuit  wrill  be  50  ohms  of  resistance  pins  33.5  ohms 
of  reactance  =  60.2  ohms  of  impedance,  so  that  the 
current  strength  passing  through  the  circuit  from  an 
alternator,  maintaining  1100  volts  effective  at  its  termi- 


nals, will  be 


1100 


=  18.27  amperes,  which  is  seen  to  be 


nearly  three  times  as  much  as  if  the -condenser  had  been 
connected  directly  with  the  alternator. 

t 


_  ,  125.7 

i  INDUCTANC 


Slec.Engineer 


FIG.  110. 


FIG.  111. 


FIG.  112. 


In  a  continuous-current  circuit,  the  drop  at  the 
terminals  of  any  resistance  R  ohms,  traversed  by 
a  current  /amperes,  is  I  R,  volts;  so,  in  an  alternating 
current  circuit,  the  drop  at  the  terminals  of  any  imped- 
ance </  ohms,  traversed  by  a  current  of  7  amperes  effec- 
tive, is  /f/  volts.  The  condenser  has  in  this  case  a  re- 
actance by  itself  of  159.2  ohms,  which,  in  the  absence  of 
inductance  or  resistance,  becomes  the  impedance  «/,  of 
the  condenser.  The  current  strength  /,  is  18.27  am- 


248 


peres,  and  the  drop  on  tlie  condenser  /  «/",  is  18.27   X 

159.2  =   29i>9   volts.     If    the   inductance-reactance  of 
125.7  ohms  could  be  separated  from  its   accompanying 
resistance  in  the  coil,  the  drop  on  the  resistance  itself 
would  be  18.27  X  50   =  913.5  volts  ;  but,  since  the  in- 
ductance and  resistance  of  a  coil  of  wire  cannot  be  sepa- 
rated, all  that  can  be  observed  is  the  drop  at  the  termi- 
nals of  the  two  coils  ;  namely,  at   the   terminals   of  the 

135.3  ohms  impedance,  as  represented  in  Fig.  Ill,  and 
in  this  case  the  pressure  at  the  terminals  of  the  resist- 
ance would  be  18.27  X  135.3  =  2472  volts.     It  follows, 
therefore,  that  a  sinusoidal  effective  E.  M.  F.  of  1100  volts, 
which  never  exceeds  1555  volts  at  the  peak  of  the  waves, 
can  produce  a  pressure  that  could  be  measured  witli  a 
suitable  voltmeter,  of  2472  volts  across  the  terminals  of 
the  resistance  coil,  and  a  further  pressure  in   series  with 
this  of  2909  volts  across  the  terminals  of  the  condenser, 
making  a  total  pressure  arithmetically  of  5381  volts;  but 
geometrically  the  sum  of  these  two  pressures  can  only 
be  1100  volts,  because  the  impressed  E.  M.  F.,  as  shown  in 
Fig.  112,  is  out  of  phase  with  the  c.  E.  M.  F'S. 

SYLLABUS. 

When  two  sinusoidal  E.  M.  F.'S  are  connected  in  series 
their  resultant  will  be  their  geometrical  sum. 

Laboratory  of  Houston  &  Kennelly, 
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A.  E.  Kennelly,  F.  R.  A.  S. 


INTERMEDIATE 

Alternating  Currents. 


257.  In  a  continuous-current  circuit,  the  reciprocal 
of  a  resistance  is  called  a  conductance  (Sec.  33).  In 
an  alternating-current  circuit,  the  reciprocal  of  an  im- 
pedance is  called  an  admittance.  If 
an  impedance  A  B,  such  as  represented 
in  Fig.  113,  of  1.5  ohms,  be  transformed 
into  an  admittance,  its  length  will  be 
the  reciprocal  of  the  length  A  B,  or 
y1^  —  0.667  mho,  and  its  inclination 
to  the  horizontal  will  be  reversed,  as 
shown  at  a  b. 

In  a  continuous-current   circuit,   as 
has  already  been  stated  (Sec.  33),  the 
joint  conductance  of  a  number  of  con- 
b*    ductances  in  parallel  is  the  sum  of  the 
Fin.  113.  separate  conductances.     In    an   alter- 

lllustra ting  Plane  Vector  .  ,  .     . 

Reciprocals.  nating-cuiTent    circuit,  the   joint  ad- 

mittance of  a  number  of  admittances  in   parallel  is  the 
geometrical  sum  of  the  separate  admittances. 


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Elec.  Engineer 


250 


258.  If  an  alternator,  Fig.  114,  producing  a  sinu- 
soidal E.  M.  F.  of  1100  volts  effective,  at  a  fre- 
quency of  125  ~,  be  connected  to  two  impedances  in 
parallel  consisting  of  (1)  a  condenser  of  10  microfarads 
capacity  and  (2)  a  coil  of  30  ohms  resistance  with  an  in- 
ductance of  0.2  henry,  the  angular  velocity  of  the  K.  M.  F. 
will  be  6.283  X  125  =  785.4  radians  per  second,  and 
the  product  of  this  and  the  capacity  of  10~r'  farad  = 
7.854  X  10~3.  The  impedance  of  the  condenser  is. 


BE 


FIG.  114. 

Illustrating  Joint  Impedance  of  Impedances  in  Parallel. 

therefore,  -  oh  rns,  in  a  down  ward  direction,  as 

'  7.854  X  10-3 

represented  bv  the  line  D  E.    The  impedance  of  the  resist- 
ance coil  will  be  the  geometrical   sum   of  the   resistance 
A  B,  of  30  ohms,  and  a  reactance  B  c,  of   785.4  X  0.2  = 
157.1  ohms,  so  that  the  impedance  is  159.9  ohms,  along 
the  line  A  c. 

The  reciprocal  of  the  condenser-impedance  D  E,  will 

have  a  length  or  0.007854  mho,  and  will   be  di- 

127.o 


251 


reeted  vertically  upwards  instead  of  downwards,  as  shown 
by  the  line  D  E,  set  off  on  a  suitable  scale.  The  acjmit- 

tance  of  the  coil  will  have  a  length     .  l        =    0.006252 

i  oy.y 

mho,  and  will  be  set  off  downwards,  at  an  angle  with  the 
horizontal,  equal  to  the  angle  CAB,  as  shown  by  the  line 
a  <?,  so  that  the  geometrical  sum  of  the  two  admittances 
a  c,  and  D  E,  will  be  a'  e,  having  a  length  of  0.002076 
mho.  This  will  be  the  joint  admittances  of  the  two  par- 
allel admittances,  and  the  joint  impedance  will  be  the 

reciprocal  of  this,   or  ---   =  481.6    ohms    marked 
0.002u76 

off  downwards.  The  current  supplied  from  the  alterna- 
tor to  this  combination  of  impedances  will,  therefore,  be 

..   =  2.284  amperes.     The  current  which  must  be 

supplied  to  the  condenser,  considered  separately,  since 
its  terminals  are  connected  directly  to  the  alternator, 

must  be  =     8.641    amperes,    and     the     current 

through  the  coil   must  be,  for  a   similar  reason, 

' 


=  6.877  amperes.  It  is  evident,  therefore,  that  the  cur- 
rent through  the  conductors  I  m  and  p  o,  will  be  2.284 
amperes. 

259.  From  the  preceding  it  appears  that  a  current 
of  2.284  amperes  can  supply  a  total  current  of 
15.518  amperes.  The  reason  is,  however,  that  while  the 
current  in  the  coil  is  lagging  considerably  behind  the 
K.  M.  F.,  the  current  in  the  condenser  has  a  considerable 
lead,  or  is  in  advance  of  the  E.  M.  F.  The  current  leav- 
ing the  condenser  at  the  moment  it  is  discharged,  is, 


252 


therefore,  able  largely  to  supply  the  current  entering  the 
coil  and  only  the  deficit  has  to  be  made  up  by  the  cur- 
rent from  the  generator,  which  is  nearly  in-step  with 
the  impressed  E.  M.  F. 

260.  In  a  continuous-current  circuit,  the  activity  is 
the  product  of  the  pressure  in  volts  and  the  cur- 
rent strength  in  amperes.  Thus,  if  a  pressure  of  100 
volts  applied  to  the  terminals  of  a  circuit,  supplies  a  cur- 
rent of  five  amjieres  through  that  circuit/the  activity  in 
the  circuit  from  the  souce  of  E.  M.  F.  will  be  500  watts. 
In  an  alternating-current  circuit,  the  activity  is  still  the 
product  of  the  pressure  and  current,  provided  that  they 
are  in-step,  that  is,  co-directed.  Thus,  a  pressure  of  100 
volts  effective,  supplied  to  the  terminals  of  an  incandes- 
cent lamp,  taking  0.5  ampere  through  it,  develops  in  the 
lamp  an  activity  of  50  watts,  because,  there  being  no  ap- 
preciable inductance  in  the  lamp  filament,  the  current 
through  the  lamp  will  be  in-phase,  or  in-step,  with  its 
impressed  E.  M.  F.  When,  however,  the  current  is  not 
in-step  with  the  electromotive  force,  the  activity  is  not 
the  simple  product  of  the  two,  but  their  co-directed  pro- 
duct. Thus,  if  the  E.  M.  F.  acting  on  a  coil  be  10  volts, 
as  shown  by  the  line  A  B,  in  Fig.  115  (1),  directed  say 
horizontally,  and  the  impedance  of  the  coil  be  2  ohms, 
so  that  the  current  A  c,  produced  in  the  coil  is  five  am- 
peres, inclined  at  an  angle  of,  say  60°,  with  the  horizon- 
tal, that  is,  lagging  60°  or  -^  period  behind  the  E.  M.  F.,  — 
then  the  projection  of  the  strength  of  the  current  upon 
the  direction  of  the  E.  M.  F.,  that  is,  upon  the  horizontal 
line,  will  be  the  length  A  DJ  which  in  this  case  is  just  half 
A  c,  or  2.5  amperes,  and  this  multiplied  by  the  pressure 
will  give  the  activity,  or  25  watts.  It  is  evident,  there- 


253 


fore,  that  the  further  the  current  lags  behind,  or  ad- 
vances before,  the  E.  M.  F.,  or,  in  other  words,  the  greater 
the  difference  of  phase  between  the  current  and  E.  M.,  F., 
the  less  will  be  the  activity,  or  rate  of  expending  energy, 
in  the  circuit  for  a  given  current  strength.  The  lag  or 
lead  of  the  current  in  a  circuit  cannot  exceed  90°  from 
the  E.  M.  F.  producing  it.  It  can  never  in  practice  ac- 
tually equal  'JO0,  for  in  such  a  case,  as  represented  in  Fig1. 
115  (2),  the  projection  of  the  current  on  the  line  ^of 
E.  M.  F.  would  vanish,  and  the  activity  in  the  circuit  would 
be  sustained  without  any  energy  being  supplied,  which, 
of  course,  is  an  impossibility. 


Kite.  Enginq&r 

t 
FIG.  115  PIG.  116. 

Illustrating  the  relations  between  E.M.F.,  Type  of  current   wave   supplied   to  a 

current,  and  activity,  in  sinusoidal  current   .     ferric  circuit  transformer  on  light  land, 
circuits. 

261,  In  calculating  the  power  in  any  alternating-cur- 
rent circuit,  the  true  activity,  divided  by  the  ap- 
parent activity,  is  called  the  power  factor,  and  in  the 
case  of  sinusoidal  currents  is  the  ratio  of  the  projection 
of  the  current1  on  the  line  of  E.  M.  F.,  to  the  current 
strength.  Thus  in  Fig.  115  (1),  the  projection  of  the 
current  being  half  the  current  strength,  the  power  fac- 
tor in  that  circuit  is  0.5 ;  or,  tile  true  activity  being  25 
watts,  and  the  apparent  activity  10  X  5  =  50  watts,  the 
power  factor  =  f|  =  0.5.  When  the  currents,  or 
E.  M.  F.'S,  are  not  sinusoidal,  the  projection  of  the  current 
strength  upon  the  line  of  E.  M.  F.  cannot  be  resorted  to, 


254 


but  in  every  case  the  ratio  of  the   true  to  the  apparent 
activity  is  the  power  factor. 

For  example,  the  waves  of  current  supplied  to  a  ferric- 
circuit  transformer  (a  transformer  whose  magnetic  cir- 
cuit is  completely  made  up  of  iron)  are  far  from  being 
sinusoidal,  when  the  transformer  is  idle,  or  on  light 
loads.  The  type  of  such  a  wave  is  shown  in  Fig.  1 1  f >, 
where  the  maximum  amplitude  or  crest  of  the  wave,  in- 
stead of  occuring  midway  between  the  zero  passages,  as 
in  a  sinusoidal  wave,  is  displaced  along  the  surface,  owing 
to  the  influence  of  hysteresis  in  the  iron,  because  a  large 
change  of  current  is  necessary  to  produce  a  small  change 
in  flux  at  the  turning  point  in  the  magnetic  cycle  (Sec.  1 54, 
Fig.  05).  It  is  difficult  to  assign  an  angle  of  lag  to  such 
a  wave ;  and,  consequently,  the  ratio  of  the  projection 
to  the  actual  current  strength  cannot  be  determined, 
while  the  apparent  and  actual  activities  in  a  circuit  can 
always  be  found  by  means  of  a  suitable  voltmeter,  am- 
meter and  wattmeter.  In  transformers,  however,  the  cur- 
rent tends  to  become  more  nearly  sinusoidal  as  their  load, 
that  is,  their  output,  is  increased,  so  that  the  waves  of  cur- 
rent supplied  by  a  sinusoidal  alternator  to  a  circuit  sup- 
plying transformers  at  different  loads  are  seldom  so  dis- 
torted as  those  shown  in  Fig.  1 1  f>. 

202.  The  power  factor  of  an  alternating  current 
transformer  with  ferric  circuit  varies  from  0.7  at 
no  load,  to,  perhaps,  0.99  in  large  transformers  at  full 
load,  but  in  aero-ferric  transformers,  whose  magnetic  cir- 
cuits are  formed  only  partly  of  iron,  the  power  factor  at 
no  load  may  be  as  low  as  0.4.  The  power  factor  of  an 
alternating-current,  synchronous  motor,  may  vary  from 
0.9  to  1.0,  and  in  an  alternating  current  induction  mo- 


255 


tor,  from  0.5,  on  light  load,  to  0.85  or  0.9  at  full  load. 
The  average  alternating-current  circuit  has  a  power  fac- 
tor of  about  0.95,  so  that  the  apparent  activity  is  only 
about  5  per  cent,  in  excess  of  the  actual  activity. 

263.  The  ratio  of  the  impedance  of  a  circuit  or  con- 
ductor to  its  resistance   is  called    its  impedance 

factor.  The  impedance  of  a  line  or  conductor  is  almost 
always  greater  than  its  resistance,  owing  to  the  induc- 
tance of  the  conducting  loop,  and  the  impedance  factor 
shows  how  many  times  greater  than  the  resistance  this 
impedance  is.  The  impedance  factor  depends  upon  the 
frequency  of  alternation  and  increases  with  the  size  of 
conductor.  Thus  at  120  ~,  the  impedance  factor  of  two 
No.  4  A.  w.  G.,  copper  wires,  suspended  in  air  parallel  to 
each  other,  at  an  interaxial  distance  of  five  feet,  is  1.6, 
so  that  the  apparent  resistance  of  such  a  pair  of  conduc- 
tors would  be  60  per  cent,  in  excess  of  their  ohmic  re- 
sistance. The  ratio  of  the  reactance  of  a  conductor  or 
circuit,  to  its  ohmic  resistance  is  called  its  reactance  fac- 
tor, and  measures  the  tangent  of  the  angle  of  lasr  or  lead 
in  the  case  of  sinusoidal  currents. 

264.  It  has  already  been  stated  (Sec.  45)  that  the  ap- 
parent resistance  of  a  rod   or  cylinder  is  greater 

for  alternating  than  for  continuous  currents.  The  reason 
for  this  is  to  be  found  in  the  fact  that  if  we  consider,  for 
example,  a  long  straight  conductor,  carrying  10  amperes, 
the  magnetic  flux  will  encircle  the  axis  of  this  wire  in  an 
alternately  right-handed  and  left-handed  direction  at  each 
alternation  of  the  current.  Of  this  flux,  perhaps  80  per 
;»ent.  will  lie  outside  the  wire,  and  the  remainder,  or  20 
per  cent.,  will  be  contained  in  the  substance  of  the  wire, 
there  being  no  magnetic  flux  at  the  axis  or  centre.  The 


256 


pulsating  magnetic  flux  induces  a  c.  E.  M.  F.  directed 
along  the  wire  and  opposing  the  establishment  of  the 
current  in  it.  While,  however,  the  central  portions  of 
the  wire  have  the  full  c.  K.  M.  F.  produced  by  all  of  this 
flux,  the  external  or  superficial  portions  have  a  c.  E.  M.  F. 
only  80  per  cent,  as  great,  since  it  is  produced  by  the 
external  flux  only.  The  result  will  be  that  the  impedance 
of  any  filament  of  wire  near  the  centre  will  he  greater 
than  that  of  a  corresponding  filament  near  the  outside, 
and  the  current  will,  therefore,  be  distributed  more 
densely  in  the  outer  layers. 

265.  The  imperfect  penetration  of  an  alternating  cur- 
rent into  the  interior  portions  of  a  conducting  wire 
is  called  the  skin  effect  of  alternating  currents.  At  high 
frequencies,  and  in  large  sizes  of  wire,  the  skin  effect  may 
be  very  considerable,  but  for  commercial  frequencies 
and  with  the  sizes  of  wire  employed  in  overhead  con- 
struction, the  impedance  due  to  skin  effect  is  very  small. 
Thus  in  the  case  of  a  No.  000  copper  wire,  carrying  cur- 
rents whose  frequency  is  14-0  ^,  the  impedance,  owing 
to  the  influence  of  imperfect  current  penetration  is  only 
1.6  per  cent,  greater  than  the  ohmic  resistance.  In  iron 
wires,  however,  this  influence  is  much  more  marked, 
owing  to  the  greater  proportion  of  magnetic  flux  existing 
within  the  substance  of  the  alternately  magnetized  wire. 
The  impedance  of  a  Xo.  7  A.  w.  G.  iron  wire  at  140  — - 
may  be  double  that  of  its  ohrnic  resistance,  owing  to  the 
effect  of  imperfect  current  penetration. 

In  telephony  the  advantage  of  copper  wires  over  iron 
wires  is  principally  ascribed  to  their  reduced  skin  effect. 

Liaboratory  of  Houston  &  Kennelly, 
Philadelphia. 


[Copyright,  1894,  by  THE  ELECTRICAL  ENGINRKK.] 
WEEKLY. 

Nn    T3  TA  v  0£    1SQ*  Price,     -    10  Cents. 

JANLARI  Jt>,  1    J5.         Subscnption,  $3.00. 

Electrical    Engineering   Leaflets, 


Prof.  E.  J.  Houston,  Ph.  D. 

AND 

A.  E.  Kennelly,  F.  R.  A.  S. 


I1MXERJYIEDIATTE   CRADE. 


266.  The  number  of  poles  on  a  continuous-current 
generator  is  largely  a  matter  of  economy  and  con- 
venience in  construction.  In  an  alternator,  the  number  .of 
poles  is  prescribed,  as  soon  as  the  frequency  and  the  number 
of  revolutions  of  the  armature  per  second  has  been  deter- 
mined upon,  since  these  two  considerations  determine  the 
frequency  of  alternation.  A  bipolar  alternator,  generates 
one  cycle  for  each  complete  revolution  of  its  armature  ; 
a  four-pole  machine,  generates  two  cycles  for  each 
complete  revolution  of  its  armature;  and  a  machine 


poles  will,  therefore,  generate  •£•  cycles  for  each  revolu- 

2 

tion  of  its  armature.  Consequently,  the  frequency  of  an 
alternator  is  -^S  cycles  per  second,  where  n,  is  the 

number  of  revolutions  of  its  armature  per  second  ;  thus 
an  alternator  of  16  poles,  making  16  revolutions  per  sec- 
ond, would  have  a  frequency  of  128  ~.  Some  inductor 

Published  by 

THE   ELECTRICAL  ENGINEER, 
203  Broadway,  New  York,  N.  Y. 

[Entered  as  «econd-class  matter  at  the  New  York,  N.  Y.,  Post  Office,  June  14,  1894.  ] 


258 


alternators,  however,  which  revolve  masses  of  soft  iron 
instead  of  wire,  produce  twice  as  great  a  frequency,  or  a 
frequency  of  n  p,  cycles  per  second. 

267.  The  character  pf  the  E.  M.  F.  wave  generated  by 
an  alternator,  depends  upon  the  dimensions  of  the 

pole  pieces  and  winding  spaces,  so  that  by  varying 
the  distance  between  the  poles,  or  their  shape,  or  the  dis- 
tance between  the  coils  on  the  armature,  as  well  as  the 
shape  of  the  space  these  occupy,  the  type  of  E.  M.  r.  wave 
may  be  varied. 

268.  In  most  alternators,  the  armature  is  revolved  in 
a  fixed  field  frame.     From  this  the  E.  M.  F.  gen- 
erated is  connected  with  the  circuit  they  supply  through 
brushes  resting  on  collector  rings,  in  lieu  of  the  commu- 
tators employed  in  continuous-current  generators.      In 
other  alternators,  however,  the  armature   is  maintained 
at  rest,  and  the  field  frame  revolved  about  it.     In  such 
machines  the  current  is  supplied  through   brushes  and 
collector  rings  to  the  magnets,  while  the  armature  is  con- 
nected directly  to  the  line.     In  still  other  forms  of  alter- 
nators, the  field  and  armature  are  both  fixed,  and  masses 
of  iron  are  used  to  vary,  by  their  revolution,  the  magnetic 
circuits  between  the  two.     Such  alternators  are  called 
inductor  alternators. 

269.  The  coils  in  alternating  armatures  are  either  of  the 
Gramme  ring,  the  drum,  the  disc,  or  the  pole  arma- 
ture type.     The  most  usual,  owing  to  its  convenience  of 
construction,  is  the  pole  type,   but  other   forms  are  in 
common  use,  especially  in  Europe.     The  armature  coils 
are  sometimes  connected  in  series  and  sometimes  in  par- 
allel-series, as  shown  in  Figs.  117  and  118.  When  wound 


259 


in  parallel-series,  twice  the  number  of  armature  turns  is 
required  for  the  same  E.  M.  r. 


FAec.  Engineer 

FIG.  118. 


SUe.  engineer 


FIG.  117.  FIG.  118.  FIG.  119. 

270.     When  a  wave  of  E.  M.  F.  or  current  is  not  a  simple 

sinusoid,  for  example,  irithe  case  of  such  a  wave  ae 

is  represented  in  Fig.  119,  it  is  sometimes  convenient  to 


SUCTANT 


FIG.  120. 


FIG.  121. 


260 


regard  the  wave  as  capable  of  being  analyzed  or  decora- 
posed  into  components,  all  of  which  are  sinusoids  ;  that  is, 
into  a  fundamental  sinusoid  and  its  harmonics.  It  can 
be  shown  that  any  periodic  wave  possessing  a  definite 
frequency,  no  matter  how  complex  its  form,  can  be  re- 
solved into  a  fundamental  sinusoidal  wave  of  the  same 
frequency,  and  a  numbef  of  ripple  chains,  or  harmonics, 
each  harmonic  having  a  frequency  some  integral  multi- 
ple of  the  fundamental  frequency.  Thus  Fig.  120,  rep- 
resents at  s,  a  sinusoidal  wave  of  E.  M.  r.  having  a  fre- 
quency of  100  ~,  and  an  amplitude  of  800  volts.  Its 
first  harmonic, — that  is,  a  sinusoidal  wave  of  double  the 
frequency,  and  which  in  this  case  has  an  amplitude  of 
400  volts,  and  starts  in  phase  with  the  fundamental, — is 
represented  at  P.  Its  second  harmonic,  having  three 
times  the  frequency  of  the  fundamental  and  in  this  case 
with  the  amplitude  of  500  volts,  also  starting  in  phase 
with  the  fundamental  is  seen  at  Q.  If  two  alterna- 
tors, respectively  producing  the  waves  s,  and  P,  were 
rigidly  coupled  on  the  same  shaft,  the  E.  M.  F.  they  would 
jointly  produce  in  the  circuit,  would  be  represented  in 
Fig.  121  where  the  wave  o  G  H  j  K,  is  the  sum  of  the  com- 
ponent waves  o  a  b  c  d,  and  o  A  B  c  v,  Fig.  120.  The  re- 
sultant wave  is  observed  to  be  asymmetrical ;  that  is  to 
say,  if  the  positive  wave  o  G  H  j  K,  Fig.  121,  be  revolved 
about  the  line  c  K,  so  as  to  be  completely  reversed  in  di- 
rection, it  will  not  coincide  with  the  following  negative 
wave  K  L  M  N  P.  This  lack  of  symmetry  is  owing  to  the 
addition  of  the  odd  harmonic ;  for,  the  first,  third,  fifth, 
etc.,  harmonics  have  the  property  that  when  added  to 
the  fundamental  wave,  either  singly  or  in  combination, 
they  produce  asymmetry  about  the  zero  line,  and  since 


261 


all  properly  constructed  alternators  produce  symmetrical 
waves  of  E.  M.  F.  and  current,  in  which  each  wave  differs 
from  its  successor  or  antecedent  in  direction  only,  such 
harmonics  do  not  exist  in  the  forms  of  wave  com- 
mercially employed. 

Similarly  if  in  Fig.  120,  the  three  waves  P,  Q,  and  s  be 
combined,  their  resultant  will  be  the  wave  shown  in 
Fig.  119,  whose  amplitude  is  about  1330  volts.  This 
wave  is  also  asymmetrical,  owing  to  the  presence  of  the 
first  harmonic. 

271.  Fig.  121  shows  the  effect  of  combining  a  funda- 
mental wave  F,  of  a  particular  amplitude  and  phase, 

with  its  second  harmonic.  F  -|-  A,  the  resultant  of  F  and 
A,  is  of  the  flat-topped  type,  while  F  -(-  B,  similarly  com- 
pounded of  F,  and  B,  is  of  the  peaked  type.  A  and  B, 
have  the  same  amplitude,  but  differ  in  phase  by  half  a 
period,  or  180°.  Both  the  resultant  waves  are  symmetri- 
cal, and  it  can  be  demonstrated  that  the  addition  to  a 
fundamental  wave  of  any  number  of  even  harmonics,  of 
any  amplitude  or  phase,  will  always  produce  a  symmetri- 
cal wave,  no  matter  how  complex  its  form.  The  flat- 
topped  or  peaked  type  of  alternating  E.  M.  F.  or  current 
may,  therefore,  be  equivalent  to  the  result  produced  by 
the  presence  of  a  prominent  second  harmonic. 

272.  When  a  complex-harmonic  E.  M.  F.  is  impressed 
upon  a  circuit,  the  current  may  be  considered  as 

the  sum  of  all  the  component  currents  which  each  com- 
ponent of  E.M.  F.,  considered  as  a  separate  alternator,  could 
independently  produce  in  the  circuit.  The  upper  har- 
monics have  so  high  a  frequency  that  the  reactance 
offered  to  them  by  inductance  in  the  circuit  produces  a 


high  impedance  to  their  current,  and,  consequently,  in 
a  circuit  containing  considerable  inductance,  the  upper 
harmonics  in  the  current  are  greatly  weakened,  so  that 
the  wave  of  current  tends  to  approach  the  fundamental 
sinusoid.  In  fact,  it  is  seldom  necessary  to  introduce 
more  than  a  second  and  fourth  harmonic  into  the  har- 
monic analysis  of  any  practical  alternating-current  wave. 
On  the  other  hand,  the  effect  of  hysteresis  in  iron  cores 
linked  with  a  conducting  circuit,  is,  as  we  have  already 
seen,  likely  to  produce  considerable  distorsion  of  cur- 
rent wave  type. 

273.  When  an  alternating  E.  M.  F.  or  current  is  spoken 
of  as  possessing  harmonics,  it  is  not,  therefore,  to 

be  inferred  that  those  harmonics  are  actually  present, 
but  that  the  type  of  curve  is  such  as  could  be  produced 
by  the  admixture  of  certain  harmonics,  with  a  funda- 
mental having  the  frequency  of  the  wave,  and  that  the 
effects  of  such  an  E.  M.  F.  or  current,  would  be  dupli- 
cated in  the  E.  M.  F.  or  current  under  consideration.  In 
fact,  with  the  alternating  E.  M.  F.'S  and  currents  in  com- 
mercial use,  the  consideration  of  harmonics  may  for 
many  purposes  be  neglected. 

274.  Two  methods  of  winding  alternators  are  in  com- 
mon use,  viz.,  the  series  and  the  parallel-series, 

as  already  shown  in  Figs.  117  and  118.  In  the  former, 
the  total  E.  M.  F.  of  all  the  coils  is  utilized,  but  the  full 
pressure  of  the  alternator  is  developed  between  the  two 
neighboring  extremities  A,  and  B.  In  the  latter,  twice  as 
many  turns  have  to  be  wound  on  the  machine  to  produce 
the  same  E.  M.  F.  as  in  the  former  case,  but  the  points  of 
maximum  pressure  are  now  as  far  removed  on  the  arma- 
ture as  possible. 


263 


Alternators  may  be  self-excited  by  commuting  a  cur- 
rent from  a  small  special  winding,  and  directing  this 
rectified  current  through  the  field  magnets.  In  almost 
all  cases,  however,  alternators  are  separately  excited  by 
means  of  a  small,  continuous- current  generator,  operated 
on  the  same  shaft  or  by  a  belt  running  from  an  arm- 
ature shaft. 

Alternators  are  frequently  compound-wound.  This 
winding  may  be  arranged  in  one  or  two  ways  ;  viz.,  either 
the  main  current  supplied  from  the  armature  is  led 
through  a  shunted  commutator,  011  the  armature  shaft, 
which  is  connected  with  a  special  winding  on  the  field 


"-I!*M_J!_    ^    3 

SECONDS   => 

Mec.  Engineer 

FIG.  122. 


Elec.  Engine 

FIG  123. 


magnets,  so  that  a  portion  of  the  outgoing  current  is 
commuted  and  sent  through  the  field  winding,  as  shown 
in  Fig.  1'22;  or  a  transformer  is  placed  in  the  path  of  the 
outgoing  current,  and  the  secondary  coil  is  connected 
through  the  commutator  with  the  field  winding,  so  that 
as  the  outgoing  current  increases  in  strength  the  field 
winding  receives  additional  excitation. 

275.     In  continuous  current  generators  the  drop  in  the 
armature  is  almost  entirely  owing  to  the  resist- 
ance of  the  armature  ;  some  little  being,  however,  due  to 
a  c.  E.  M.  F.  of  self-induction  in  the  coils  undergoing  com- 


264: 


mutation,  and  some  to  armature  reaction  and  c.  M.  M.  F. 
In  an  alternator,  however,  the  drop  is  not  only  due  to 
the  resistance,  but  also  to  the  reactance  of  the  armature, 
so  that  the  drop  is  increased  from  IP,  to  /-/,  volts ;  J,  be- 
ing the  impedance  of  the  armature.  In  addition  to  this, 
there  is  usually  some  drop  due  to  armature  reaction  and 
c.  M.  M.  F.,  but  as  there  is  no  commutator,  there  is  no  loss 
due  to  commutative  action. 

SYLLABTTS. 

The  frequency  which  an  alternator  has  to  supply  de- 
termines the  number  of  its  poles  when  its  speed  of 
rotation  is  given. 

The  character  of  the  E.  M.  F.  wave  of  an  alternator  de- 
pends upon  the  relative  size  and  spacing  of  the  poles 
and  armature  winding. 

Most  alternators  revolve  their  armatures  ;  some  revolve 
their  fields,  and  a  few  revolve  a  mass  of  iron,  forming  a 
portion  of  their  magnetic  circuit. 

Alternating  waves  of  E.  M.  F.  and  current,  when  not 
strictly  sinusoidal,  may  be  resolved  into  a  fundamental 
and  a  member  of  harmonics. 

The  combination  of  a  fundamental  with  any  of  its  odd 
harmonics  produces  an  asymmetrical  wave  with  respect 
to  the  zero  line,  but  its  combination  with  any  of  its 
even  harmonics  produces  a  symmetrical  wave. 

Laboratory  of  Houston  &  Kennelly, 
Philadelphia. 


[Copyright,  1894,  bv  THK  ELECTRICAL  ENGINEER.] 
WEEKLY. 

No    34  FTTRT?TTAT?V  O    1QQK  Price,     -    10  Cents. 

4  L  Subscription,  S3.00. 

Electrical    Engineering   Leaflets, 


Prof.  E.  J.  Houston,  Ph.  D. 

AND 

A.  E.  Kennelly,  F.  R.  A.  S. 


INTERMEDIATE 

ALTERNATORS. 


276.  Alternators  employed  for  incandescent  lighting 
usually  have  a  frequency  of  from  125  ~  to  133 
~,  and  should  have  a  frequency  above  35  ~  in  order  to 
ensure  steadiness  of  the  light.  Below  30  ~  incandes- 
cent lamps  appreciably  flicker,  showing  pulsations  in  the 
light  emitted  corresponding  to  the  pulsations  of  the  cur- 
rent, especially  with  high  pressure,  high  efficiency  fila- 
ments, which  have  necessarily  a  very  small  cross  section 
and  a  high  temperature. 

A  type  of  alternator  suitable  for  incandescent  lighting 
at  a  frequency  of  133  ~~,  is  shown  in  Fig.  124.  This 
machine  has  a  commutator  provided  at  M,  for  rectifying 
the  induced  current  through  the  compound-wound  field 
magnets,  so  as  to  maintain  a  constant  E.  M.  F.  at  collector 
rings  R,  R',  under  all  conditions  of  load.  It  has  28 
poles  and  makes  571  revolutions  per  minute  with  a  ca- 
pacity of  450  K  w. 

The  E.  M.  F.'S  supplied  by   such   alternators   are   1000, 

Published  by 

THE  ELECTRICAL  ENGINEER, 
903  Broadway,  New  York,  N.  Y. 

[Entered  as  second-class  matter  at  the  New  York,  N.  Y.,  Post  Office,  June  14,  1894.] 


266 


2000  or  3000  volts  effective,  representing  a  maximum 
E.  M.  F.  in  each  cycle  of  about  1414,  2828,  or  4242  volts, 
on  the  assumption  that  the  waves  of  E.M.  F.are  sinusoidal. 
The  effective  pressures  at  machine  terminals  may  l>e  five 
to  fifteen  per  cent,  in  excess  of  the  E.  M.  F.'S  to  be  sup- 
plied in  the  mains  in  order  to  allow  for  drop  in  con- 
ductors. 


FIG.  124. 

Alternator  for  Incandescent  Lighting. 

277.  In  all  incandescent  alternators,  the  inductance 
and,  therefore,  the  reactance  of  the  armature  is 
kept  as  low  as  conveniently  possible,  so  as  not  to  obtain 
an  unduly  large  impedance  in  the  armature  of  the  ma- 
chine, and  so  as  to  prevent  an  excessive  drop  in  the 
armature  under  load.  In  alternating  arc  generators, 
however,  the  armatures  are  required  to  vary  automati- 


267 

callj  the  E.  M.  F.  at  the  collector  rings  in  conformity  with 
the  number  of  lamps  that  are  operated  in  series  in 
the  circuit  through  the  medium  of  their  respective  trans- 
formers. This  is  accomplished  by  giving  to  the  armature 
a  large  inductance  and  consequent  reactance,  and  also 
by  arranging  for  a  powerful  reactive  effect  between  the 
c.  M.  M.  F.  in  the  armature  and  the  M.  M.  F.  of  the  Held. 
By  this  means  the  drop  of  pressure  in  the  armature,  and 
the  reactive  M.  M.  F.,  keep  the  pressure  at  collector  rings 
down  to  that  required  for  supplying  under  all  conditions 
of  load,  a  practically  uniform  current  through  the  line. 

21  &.  Alternators  supplying  incandescent  or  arc  lamps, 
furnish  a  single  alternating  current  through  one 
pair  of  mains  from  the  collector  rings.  Such  a  current 
is  capable  of  driving  a  similar  alternator  as  a  motor,  but 
only  when  the  motor  is  in  step  with  the  alternator.  Such 
motors  can  either  not  be  started  at  all,  or  can  only  be 
started  from  rest  under  light  load,  but  once  in  step  with 
the  generator  will  run  under  full  load  from  its  current. 
These  motors  are  called  synchronous  motors.  In  order 
to  employ  an  alternating  current  motor,  capable  of 
being  started  with  full  torque  from  rest,  which  is  the 
requirement  of  most  machinery,  multiphase  currents 
have  at  present  to  be  employed  ;  that  is,  two  or  more 
currents,  differing  in  phase  by  different  amounts,  require 
to  be  simultaneously  sent  through  the  motor  in  different 
circuits.  At  present  there  are  only  three  varieties  of 
multiphase  currents  in  commercial  use;  namely,  diphase, 
triphase  and  monoeyclic. 

279.     Two   alternating  E.  M.  F.'S   are    called  diphase 
E.  M.  F.'S,  when  they  have  the  same  frequency,  mag- 
nitude and  wave  character,  but  differ  in  phase  by  a  quarter 


268 


Cjcle,or90°,beingvtherefore,tw  quart  rutur^.  Such  K.M.F.'S 
are  as  shown  in  Fig.  1:25,  where  o  A,  indicates  an  E.  M.  F. 
of  lino  volts  rotated  at  a  definite  angular  velocity  about 
the  point  o,  but  always  in  quadrature  with  an  equal  E.M.F. 
o  B,  which  rotates  around  o,  with  it,  so  that  when  o  A, 
has  its  full  length  the  projection  of  o  B,  on  a  horizontal 
line  vanishes.  When  o  A,  reaches  the  position  shown  in 
Fig.  l!2«'» ;  namely,  after  |  of  a  period  has  elapsed,  the  pro- 
jection of  o  A',  on  the  horizontal  line  will  be  o  a',  or  778 
volts,  and  the  projection  of  o  B,  still  at  right  angles  to  o  A, 
will  be  o  by  or  778  volts  negative.  It  is  evident,  there- 

B 


6-       -778 


FIG.  125.  FIG.  126. 

Diagtam  of  Diphase  E.  M.  F.'S.  Diphase  E.  M    F.  Diagram. 

fore,  that  when  one  E.  M.  F.  has  its  maximum,  the  other 
E.  M.  F.  has  its  zero. 

The  current,  which  these  two  E.  M.  F.\S  will  send 
through  independent  circuits,  will  also  be  in  quadra- 
ture, if  the  impedances  of  those  circuits  are  equal ;  for, 
the  lag  of  each  current  behind  its  own  E.  M.  F.  will 
be  the  same  in  each  circuit.  In  some  cases,  four  wires 
and  two  separate  circuits  are  employed  for  the  dis- 
tribution of  diphase  currents  as  shown  in  Fig.  127, 
while  in  other  cases  three  wires  are  employed,  one  wire 
forming  a  common  return,  as  in  Fig.  128.  Each  circuit, 


considered  separately,  is  an  ordinary  uniplia.se  circuit  in 
which  incandescent  lamps,  arc  lamps  or  synchronous 
motors  can  he  operated,  but  the  combination  of  the  two 
currents  enables  non-synchronous  or  inductive  motors  to 

n 


Etec.  Engineer 

FIG.  127. 

Biphase  Connections,  Separate  C'iicuits. 

l>e  operated.  In  Fig.  1.28,  the  E.  M.  F.  between  neighbor- 
ing wires  is  seen  to  he  2000  volts  effective,  while  be- 
tween outside  wires,  the  E.  M.  F.  is  2828  volts  effective, 
and  this  will  be  true  whether  the  E.  M.  F.'S  are  sinusoidal 
or  not;  for,  as  shown  in  Fig.  125,  the  E.  M.  F.,  A  E,  is 
1.414  times  greater  than  either  o  A,  or  on,  by  geometry. 
Diphase  E.  :sr.  F/S  are  generated  by  two  sets  of  coils  so 
wound  on  the  armature,  with  respect  to  the  field  poles, 
that  the  E.  M.  F.  generated  in  one  is  90°,  or  £  cycle,  ahead 
of  the  E.  M.  F.  generated  in  the  other. 


Eltc.Engine«r 


FIG.  128. 

Diphase  Connections,  Interconnected  Circuits. 

280.     Three  alternating  E.  M.  F.'S  are  called  triphase 

E.  M.  F.'S,  when  they  have   the   same  frequency, 

magnitude  and  wave  character,  but  differ  in  phase  £  cycle 

or  120°.     Such  a  system  of  E.  M.  F.'S  is  represented  in 


270   . 


Fig.  129  where  o  A,  o  B,  and  o  c,  are  three  triphase 
K.  M.  F.'S,  each  of  1000  volts  effective,  revolving  together 
about  the  point  o,  with  a  definite  angular  velocity. 

281.  Triphase  E.  M.  F.'S  are  generated  by  three  sets  of 
coils  so  wound  on  the  armature  with  respect  to 
the  field  poles,  that  the  E.  M.  F.'S  in  them  are  120°  apart. 
There  are  two  methods  of  connecting  the  windings  ex- 
ternally ;  namely,  the  star-rnMliod,  indicated  in  Fig.  130, 
where  the  three  windings  are  brought  to  a  common  con- 
nection o,  and  the  triangular  method  represented  in  Fig. 
131,  where  the  three  windings  are  connected  in  one  loop 

B  B_ 

R. 


FIG.  129. 

Triphase  K.  M.  F.  Diagram. 


. 

Elec.  JKnffineer 

FIG.  130.  FIG.  131. 

Star  Triphase  Winding.      Triangle  Triphase  Winding. 

or  series  D  E  F.  Whichever  method  is  adopted  the 
E.  M.  F.  is  always  measured  between  any  two  of  the  three 
terminals  A  B  c,  or  D  E  F.  In  the  star  winding,  the 
E.  M.  F.  between  any  two  terminals  as  A  and  c,  Fig.  129, 
is  1732  volts  effective  or  1.732  times  the  E.  M.  F.  in  the 
winding  o  A,  o  B,  or  o  c,  as  is  evident  from  the  geometry 
of  the  figure,  so  that  if  the  E.  M.  F.  between  three  termi- 
nals is  1732  volts,  that  between  any  terminal  and  the 
common  connection  is  1000  volts.  On  the  contrary, 
when  connected  in  the  triangular  system,  the  E.  M.  F.  be- 
tween terminals  is  the  E.  M.  F.  of  the  winding.  The  out- 


271 


put,  however,  of  a  machine  will,  under  both  conditions, 
he  the  same,  and  in  fact  will  be  the  same  whether  the 
machine  he  divided  in  three  parts  connected  in  triphase, 
or  into  a  single  winding  and  worked  Uniphase. 

282.  A  recent  combination  of  the  uniphase  and  mul- 
tiphase systems  is  called  the  monocyclic  system. 
This  system  is  intended  to  he  a  Uniphase  system  in  so 
far  as  regards  electric,  lighting  over  an  extended  area  by 
two  wires,  but  when  multiphase  motors  are  to  be  driven, 
a  third  and  smaller  wire  called  \\\v  power  wire  is  employed 
carrying  a  special  pressure  to  such  multiphase  motors. 


•A 

Elee.  Engineer 

FIG.  132. 

Mouocyclic  E.  M.  F.  Diagram. 

The  arrangement  of  E.  M.  F/S  in  a  monocyclic  genera- 
tor is  represented  in  Fig.  132,  where  o  A,  is  the  principal 
E.  M.  F.  of  the  generator  and  is  here  represented  as  of  2000 
volts  effective  E.  M.  F.,  revolving  at  definite  angular 
velocity  about  the  extremity  o.  This  i:.  M.  F.,  connected 
to  two  collector  rings,  furnishes  a  uniphase  current  for 
incandescent  and  arc  lighting,  and  also  for  synchronous 
motors.  A  separate  winding  of  smaller  cross-sectional 
area  and  fewer  turns,  produces  the  E.  M.  F.,  B  c,  of  577  volts 
effective,  which  is  connected  between  a  third  collector 
ring  and  the  middle  of  the  principal  winding  o  A.  This 
K.  M.  F.  is  arranged  to  be  generated  in  quadrature  with 
o  A,  as  shown  in  the  figure.  Between  terminals  o,  ancj 


272 


A,  there  will  thus  be  an  E.  M.  F.  of  2000  volts,  between 
o,  and  c,  1154  volts,  leading  o  A,  by  30°  and  between  c, 
and  A,  1154  volts,  lagging  30°  behind  o  A,  consequently 
o  c  and  c  A,  are  separated  in  phase  by  60°.  The  power 
wire  from  c,  being  carried  to  the  premises  where  an  in- 
duction motor  is  to  be  operated,  two  transformers  are 
installed  each  for  half  the  power  required.  One  trans- 
former is  connected  to  the  wires  o  and  o,  as  shown  in 
Fig.  133,  and  the  other  to  the  terminals  A  and  c.  The 
E.  M.  F.  induced  in  the  secondary  winding  of  these  trans- 


nsww 
1 

PRIMARY 


lO          1154 

PRI 


FIG.  133. 


Monocyclic  Triphasc  Transformer 
Connections. 


FIG.  134. 

Combination  of  Secondary  Monocycl.c 
E.  M.  F.  into  Triphase  System. 


formers  may  each  be,  say,  100  volts  effective,  but  a  rela- 
tive angular  position  of  o  c  and  c  A,  in  Fig.  132,  or  o'  c' 
and  c'  A',  in  Fig.  134.  By  reversing  the  connection  of 
the  second  E.  M.  F.,  c'  A',  we  obtain  an  E.  M.  F.  c'  A", 
Fig.  134,  so  that  the  three  terminals  of  the  two  trans- 
formers o',  c'  and  A'',  have  between  them  three  triphase 
E.  M.  F.'S,  each  equal  to  100  volts,  as  shown  in  Fig.  134, 
and  to  these  terminals  the  triphase  motor  wires  are  at- 
tached. 


Laboratory  of  Houston  &  Kennelly, 
Philadelphia. 


(.Copyright,  1894,  by  THE  ELECTRICAL  ENGINEER.  } 
WEEKLY. 


No.  85.  FKHBUAKY  9,  1895.          ggV^  Cent*. 

Electrical   Engineering   Leaflets, 


Prof.  E.  J.  Houston,  Ph.  D. 

AND 

A.  E.  Kennelly,  F.  R.  A.  S. 


INTERMEDIATE   GRADE. 

Alternating  Cnrrent  Transformers. 

283.  An  alternating-current  transformer  consists  es- 
sentially of  an  induction  coil  in  which  an  alter- 
nating E.  M.  F.  is  induced  in  a  secondary  circuit  b}^  the 
variations  of  an  alternating  current  in  the  primary 
circuit. 

Suppose  that  a  laminated  ring  of  iron  wire  cc  c,  Fig. 
135,  be  wrapped  with  a  primary  coil  P,  and  an  alternat- 
ing E.  M.  F.  of  1000  volts  be  impressed  on  its  terminals. 
If  the  coil  has  500  turns  and  a  resistance  of  7?,  ohms, 
then  a  certain  effective  current  strength  /,  wrill  pass 
through  the  coil.  Since  the  current  is  alternating,  the 
M.  M.  F.  it  produces  sets  up  an  alternaing  flux  through 
the  coils  and  establishes  in  it  a  c.  E.  M.  F.  The  geometrical 
sum  of  this  c.  E.  M.  F.  and  the  drop,  will  be  1000  volts ; 
thus,  if  the  resistance  7?,  be  two  ohms,  and  the  current 
strength  /,  one  ampere,  the  impedance  of  the  coil  will 
be  1.000  ohms,  and  the  geometrical  sum  of  the  c.  E.  M.  F. 
and  the  drop  of  2  volts,  will  be  equal  to  the  E.  M.  F.  of 

Published  by 

THE  ELECTRICAL  ENGINEER, 
903  Broadway,  New  York,  N.  Y. 

[Entered  as  second-class  matter  at  the  New  York,  N.  Y.,  Post  Office,  June  14,  1804.] 


274 


1000  volts  at  the  terminals.  The  c.  E.  M.  r.  must  there- 
fore be  very  nearly  1000  volts. 

284.  If  now  a  secondary  coil  s.  be  wound  on  the  ring 
as  shown,  the  flux  from  the  primary  coil  may, 

neglecting  leakage,  be  considered  as  passing  entirely 
through  the  secondary  coil.  If  the  number  of  turns 
in  the  secondary  coil  be  50,  the  E.  M.  F.  induced  in  it  will 
be  very  nearly  -/^ths  of  that  at  the  primary  terminals, 
100  volts.  If  the  secondary  circuit  be  opened,  the 
presence  of  the  secondary  coil  has  no  effect  upon  the 
primary  circuit,  but  if  the  secondary  coil  be  closed 
through  a  resistance,  a  current  will  flow  through  the 
secondary  circuit,  and  will  produce  a  M.  M.  F.  in  the 
magnetic  circuit,  counter  to  the  M.  M.  F.  of  the  primary 
current.  The  primary  M.  M.  F.  is,  therefore,  weakened, 
and  the  c.  E.  M.  F.  in  the  primary  coil  weakened,  and 
the  impedance  in  the  primary  coil  being  reduced,  an 
increased  current  strength  flows  through  it  from  the 
primary  mains.  This  increase  in  current  is  sufticient, 
under  the  new  conditions,  to  re-establish  the  flux  and 
c.  E.  M.  F.  required  in  the  primary  circuit.  As  the  load 
in  the  secondary  circuit  is  increased,  the  impedance  of 
the  primary  circuit  diminishes,  and,  not  only  does  the 
primary  current  increase,  but  it  comes  more  nearly  into 
phase  with  the  primary  impressed  E.  M.  F.,  that  is,  both 
the  current  strength  and  the  power  factor  increase.  In 
other  words,  an  alternating-current  transformer  is  self- 
regulating,  under  all  variations  of  load,  up  to  the  limit 
of  the  apparatus. 

285.  In  the  alternating-current  transformer  shown  in 
Fig.   135,  the  primary    and    secondary  coils  are 

wound  on  the  outside  of  the  iron  wire  ring  forming  the 


275 


magnetic  circuit.  This  arrangement  is  objectionable  in 
practice  on  account  of  leakage,  as  illustrated  in  Fig.  136. 
Preferable  forms  are  shown  in  Fig.  137,  where  .the  pri- 
mary and  secondary  coils  are  brought  nearer  together 
and  where  they  are  more  closely  surrounded  by  iron, 
thus  reducing  the  leakage.  The  flux  paths  are  roughly 
indicated  by  the  arrows.  Other  forms,  in  which  the 
iron  lies  outside  the  coils,  are  shown  in  Figs.  138  and 
139,  where  the  primary  and  secondary  terminals  are 
represented  by  the  letters  p  p,  and  s  s,  respectively. 
Here  the  primary  and  secondary  coils  are  surrounded  by 
U-shaped  stampings  of  sheet  metal,  alternately  placed 


FIG.  135.  FIG.  136. 

above  and  below  so  as  to  produce  a  thick  and  short  mag- 
netic circuit.  The  leakage  in  such  a  transformer  is  com- 
paratively small,  but  the  transformer  has  to  be  entirely 
dismantled,  in  order  to  replace  an  injured  coil.  Still  an- 
other form  is  represented  in  Fig.  140,  where  two  coils 
c  c  arid  c  <?,  are  clamped  together  by  a  laminated  iron 
frame  i  i,  with  a  laminated  core  passing  through  the 
centre  of  the  coils. 

286.     The  output  of  a  transformer  is  limited  either 

by  the  amount  of  drop  in  the  secondary  coil,  or  by 

the  elevation  of  temperature  of  the  apparatus  under  full 

load.     A  transformer  of,  say  from  1  KW.  to  50  KW.  capa- 


276 


city,  should  not  have  more  than  4  per  cent,  drop  at  second- 
ary terminals  when  the  primary  pressure  is  maintained 
constant,  if  the  apparatus  is  heavily  overloaded,  and 
especially  if  the  design  is  inferior,  the  drop  at  the  sec- 
ondary terminals  will  become  more  than  can  be  per- 
mitted for  the  purposes  of  incandescent  lighting. 

The  heating  of  a  transformer  arises  from  expenditure 
of  energy  in  the  primary  and  secondary  circuits,  of  the 
type  /2  7?,  and  in  eddy  currents  in  the  conductor  and 
core,  and  from  hysteresis  in  the  iron  core.  The  elevation 
of  temperature  of  the  transformer,  at  full  load,  should 


FIG.  138. 


not  be  over  50°  C.  Good  transformers  are  designed  so 
that  the  temperature  elevation  shall  not  exceed  40°  C. 
Since,  in  large  transformers  the  surface  which  permits 
the  escape  of  heat  is  relatively  much  smaller  than  in 
small  transformers,  artificial  means  are  required  to  pre- 
vent undue  heating. 

287.     If  there  were  no  losses  of  energy  in  a  trans- 
former, it  is  evident  that  the  activity  delivered  in 
the  secondary  circuit  would  be  equal  to  the  activity  ab- 
sorbed at  the  primary  terminals,  and  that   the  exciting 


277 


or  magnetizing  primary  current  at  no  load  would  be 
in  quadrature  with  the  impressed  E.  M.  F.  In  all 
transformers  the  principal  loss  is  due  to  hysteresis,  which 
is  practically  the  same  at  all  loads,  while  the  losses  due 
to  /2  7?,  in  the  conductors,  are,  of  course,  dependent  upon 
the  output.  If,  therefore,  we  measure  the  expenditure 
of  activity  in  the  primary  circuit  of  a  transformer,  when 


FIG.  139. 


FIG.  140. 


its  secondary  circuit  is  open,  and  we  know  the  resistance 
of  the  primary  arid  secondary  coils,  \ve  can  readily  esti- 
mate the  amount  of  loss  which  will  take  place  at  any 
given  load.  The  losses  by  eddy  currents  are  practically 
constant  at  all  loads,  and,  by  a  proper  lamination  of  the 
iron  core,  may  be  rendered  practically  negligible.  It 
is  usual  to  employ  charcoal  iron  or  soft  steel  for  trans- 
former plates,  having  a  thickness  of  about  0.01 5". 


278 


288.  The    efficiency    of    alternating    current  trans- 
formers increases  with  their  size  and   varies  with 

their  load.  Large  transformers,  of  say  50  KW.  capacity, 
have  an  efficiency  of  more  than  0.98  at  full  load,  and 
about  0.95  at  quarter  load,  while  a  small  transformer,  of 
say  0.5  KW.  capacity,  for  about  10  incandescent  lights, 
may  have  a  full  load  efficiency  of  0.9,  and  0.7  at  quar- 
ter load.  Practically,  however,  the  most  important 
factor  in  the  efficiency  of  a  transformer  is  its  all  day  or 
average  efficiency,  that  is,  the  ratio  of  the  total  output  to 
the  total  in-take  during  the  24  hours.  Most  transformers 
employed  in  incandescent  lighting  have  to  be  operated  at 
no  load  for,  perhaps,  18  hours  in  the  24.  Here,  the  loss 
in  hysteresis  is  an  important  consideration.  It  is  evident 
that  it  may  often  be  more  economical  to  supply  incan- 
descent lighting  from  a  few  large  transformers,  with  low 
pressure  mains,  instead  of  from  a  number  of  small 
transformers,  each  connected  to  its  own  consumption 
circuit,  since  the  all-day  efficiency  of  a  large  transformer 
may  be  0.96,  while  that  of  a  number  of  small  trans- 
formers may  average  only  0.75. 

289.  The  power   factor   of   a   transformer   depends 
upon  its  size,  its  load,  and  on  the  character  of  its 

load.  For  an  Unloaded  transformer  the  power  factor  is 
usually  about  0.7,  so  that  the  activity  absorbed  is  only 
about  70  per  cent,  of  the  product  of  the  volts  and  am- 
peres at  primary  terminals.  At  a  full  load  of  incandescent 
lamps,  i.  e.,  with  an  non-inductive  load,  the  power  fac- 
tor may  be  0.99.  When,  however,  the  load  on  the  trans- 
former is  inductive,  as,  for  example,  when  motors  are 
operated  in  the  secondary  circuit,  the  power  factor  will 
not  only  be  comparatively  small  in  the  secondary  cir- 


2Y9 


cnit,  but  will  also  be  appreciably  reduced  in  the  primary 
circuit.  Thus,  the  power  factor  in  the  primary  circuit 
of  a  large  transformer  might  be  0.99  at  full  non-induc- 
tive load,  but  would,  perhaps,  be  only  0.9  at  full  induc- 
tive load.  The  effect  of  an  inductive  load  is  not  only  to 
diminish  the  power  factor,  -but  also  to  increase  the  drop 
at  secondary  terminals. 

290.  The  frequency  of  alternation   at   which   trans- 
formers are  operated  has  an  important  influence 

upon  their  size  and  efficiency.  Since  the  K.  M.  F.  induced 
depends  on  the  rate  of  change  of  the  flux,  the  greater  the 
frequency  the  greater  will  be  the  E.  M.  v.  induced  for  a 
jriven  flux,  and,  consequently,  the  smaller  will  be  the 
flux  required  for  a  given  c.  E.  M.  F.  The  size  and  weight 
of  a  transformer  can,  therefore,  be  reduced  within  cer- 
tain limits  by  increasing  the  frequency,  just  as  the  size 
and  weight  of  a  generator  or  motor  can  be  reduced  by 
increasing  its  speed  of  revolution  for  a  given  output. 

291.  The  pressures  commercially  employed  in  alter- 
nating-current primary  circuits  are   usually  1000 

or  2000  volts,  and  in  some  cases  as  high  as  10,000  volts, 
while  in  the  secondary  circuit  the  E.  M.  F.  is  usually  50  or 
100  volts,  the  secondary  coils  being  usually  in  two 
halves,  each  for  50  volts,  and  capable  of  being  connected 
either  in  parallel  or  in  series. 

Since  any  failure  in  the  insulation  existing  between 
primary  and  secondary  circuits  in  a  transformer  brings 
the  primary  pressure  into  the  consumption  circuit,  and, 
therefore,  renders  the  secondary  circuit  dangerous  to 
handle,  the  insulation  between  the  coils  themselves  and 


280 


between  the  coils  and  iron  frame  lias  to  be  carefully  pro- 
vided for  in  manufacture. 

On  account  of  the  high-pressure  connections,  trans- 
formers are  generally  placed  outside  the  building  they 
are  intended  to  supply  and  secondary  wires  carried  from 
such  point  into  the  building. 

Laboratory  of  Houston  &  Kennelly, 
Philadelphia. 


, 


INDEX. 


Activity  of  Alternating-Cur- 
rent Circuit 252,  253 

Activity  of  Voltaic  Arc.  2 17,  218 
Activity  or  Unit  of  Power, 
International    Definition 

Of, 12 

Admittance,  Definition  of. .  249 

Advantages  Possessed  by 
Electric  Motor 188,  189 

Aero-Ferric  Circuit,  Elec- 
tric Analogue  of 109 

Aero- Ferric  Circuit  With 
Leakage,  Electric  Ana- 
logue of ii 

Aero-Ferric  Magnetic  Cir- 
cuit    100 

Aero-Ferric  Magnetic  Cir- 
cuit, Diagram  of 102 

Air-gap 103 

All-day  Average  Efficiency 
of  Alternating-Current 
Transformers. 278 

All-night  Arc  Lamps 224 

Alloys,  Temperature  Co- 
efficient of 26 

Alternating  and  Fluctuat- 
ing Currents,  Difference 
Between 233 

Alternating  Circuit,  Appli- 
cation of  Ohm's  Law  to..  244 

Alternating-Current  Arc, 
Distribution  of  Light  in..  231 


Alternating-Current  Arcs. .  230 

Alternating-Current  Cir- 
cuit, Activity  of 252,  253 

Alternating  Current  Cir- 
cuit, Impedance  Factor  of  255 

Alternating-Current  Cir- 
cuit, Power  Factor  of. ...  253 

Alternating-Current  Cir- 
cuit, Quantities  Entering 
Into  Value  of  Impedance 
of 244 

Alternating-Current  Cir- 
cuit, Reactance  Factor  of  255 

Alternating-C  urrent,  Defini- 
tion of 14,  233 

Alternating-Current,  Heat 
Generated  by 198 

Alternating-Current  Trans- 
former, Construction  of. .  273 

Alternating-Current  Trans- 
former, Self-Regulating 
Character  of 274 

Alternating-Current  Trans- 
formers  273  to  280 

Alternating-Current  Trans- 
tormers,  All- day  Average 
Efficiency  of 278 

Alternating- Current  Trans- 
formers, Efficiency  of.  . .  278 

Alternating-Current  Trans- 
formers, Limitation  of 
Output  of 275,  276 


282 


INDEX. 


Alternating-Current  Trans- 
formers, Power- Factor 

of 278,  279 

Alternating-Current  Trans- 
formers, Types  of 275 

Alternating  Currents  233  to  248 
Alternating   Currents,  De- 
finition of. :     45 

Alternating-E.  M.  F.,  Defini- 
tion of 14 

Alternating    Electric    Cur- 
rent, Heating  Effect  of  .     239 
Alternation,  Amplitude  of.   239 
Alternation,  Definition  of. .   238 

Alternator,  Bipolar 257 

Alternators 257  to  272 

Alternators,  Compound  . . .  263 
Alternators,  Definitions  of  237 
Alternators,    E.  M.  F.    Pro- 
duced by 265,  266 

Alternators  for  Incande- 
scent Lighting,  Frequen- 
cies of 265 

Alternators,  Inductor.  .257,  258 
Alternators,  Parallel-Series  262 
Alternators,  Self-Excited. .   263 
Alternators,  Separately-Ex- 
cited    263 

Alternators,  Series  Connec- 
tion of ....  241 

Alternators,  Series-wound.  262 
Alternators,       Sinusoidal, 
Connection  of,  In  Quad- 
rature.    243,  244 

Alternators ,  Sinusoidal , 
Connection  of,  In  Step  . .  242 

Ammeters 46 

Ammeters,  Weston 46 

Ampere,  Definition  of  ....     43 
Ampere-Hour,  Definition  of    44 


Ampere,  International,  De- 
finition of  43 

Ampere-Meters 46 

Ampere-Turns,  Definition 
of 94 

Apparatus ,  Commercial , 
Tables  of  Resistance  of. . 

37,   38,   39 

Apparent  Insulation  of  Tel- 
egraphic Line 29 

Arc  Lamp,  All-Night 224 

Arc  Lamp.  Mean  Spherical 

Candle  Power  of 221 

Arc  Lamp,  Use  of   Shunt 

Magnet  in 226 

Arc  Lamp,  Varying  Posi- 
tion of  Carbon  Electrodes 

in 222,  223 

Arc    Lamps ,    Diagram    of 

Feeding  Mechanism  in. .   226 
Arc  Lamps,  Double-carbon  224 
Arc   Lamps,   Feeding   Me- 
chanism of 222 

Arc  Lamps,  Focusing 223 

Arc  Lamps,  Shunt  and 
Series  Winding  for  Lift- 
ing Magnets 227 

Arc  Lamps,  Series-Connec- 
tion of 227,  228 

Arc  Lighting 217  to  232 

Arc  Lights,  Circumstances 
Affecting  Steadiness  of. .  220 

Arc  Lights,  Cost  of 228 

Arc,  Voltaic,  Activity  of  217,  218 
Arc,  Voltaic,  Definition  of  .  217 
Arc,  Voltaic,  Expenditure 

of  Energy  in 218 

Armature,  Conductor  of , 
Rule  for  Calculating 
E,  M.  F.  Produced  in 134 


INDEX. 


283 


Armature,  Conductors,  Cal- 
culation of  E.  M.  F.  Gener- 
ated in 131 

Armature,  Drum-Wound ..   130 
Armature  of  Dynamo  Elec- 
tric Machine 130 

Armature  of  Dynamo , 
Means  for  the  Dissipa- 
tion of  Heat  Generated 

in 147 

Armature  of  Dynamo,  Ven- 
tilation of 148 

Armature,  Reaction  of. ...  118 
Armature,  Ring.  Wound ...   130 

Arms  of  Bridge 33 

Astatic  System 48 

Attractions  and  Repulsions, 
Electrodynamic ....  167,  168 

Attractive  Magnets 115 

Automatic  Cut-out  for  Arc 
Lamps 226,  227 

Balance,  Arms  of 33 

Balance,  Wheatstone's.  .31, 

32,    33,     34 
Bases     for      Incandescent 

Lamps 204,  205 

Battery  Installation,  Re- 
quirements for  Minimum 

Costof 86 

Battery,  Voltaic,  Definition 

of 14 

Begohm,  Definition  of 20 

Bichromate  Voltaic  Cell .  73,     74 
Bichrom,  Definition  of.   . .       20 

Bipolar  Alternator 257 

Blackening  of  Chamber  of 
Incandescent  Lamp, 

Cause  of 211 

Bougie-Decimale 206 


Bridge,  Arms  of 33 

Bridge,  Wheatstone's. .  .31, 

32,    33,     34 

Building-up     of    Dynamo- 
Electric  Machine 158 

Bunsen  Voltaic  Cell,  E.  M.  F. 

<tf 75 

Bus  Bar,  Definition  of. ..62,     63 

Callaud  Voltaic  Cell 77 

Calorie,    Gramme-,  Defini- 
tion of 193 

Calorie,  Lesser,  Definition 

of 193 

Candle-Foot,  Proposed  Unit 

of  Illumination 209 

Candle  Power,   Horizontal, 

for  Arc  Lamps ...  222 

Candle-Power  of  Lamp 206 

Candles,  Jablochkoff 222 

Capability,    Electrical,     of 

Voltaic  Cell 83,     84 

Capacity    of     Alternating- 
Current  Circuits 244 

Capacity-Reactance 245,  246 

Car  Heater,  Electric,  Cur- 
rent Required  for 197 

Carbon  Electrodes,  Manu- 
facture of 219 

Carbon    Electrodes,   Vary- 
ing   Position   of,  in  Arc 

Lamps  222,  223 

Carbon  Rheostat 36 

Carbons,   Cored,    for    Arc 

Lights 220 

Carbons,  Electro-Plated,for 

Arc  Lamps 220 

Carcel  Lamp 206 

Carcel-Metre,    Proposed 
Unit  of  Illumination 209 


284 


INDEX. 


Carrying  Capacity,  Safe,  of 

Copper  Wires 199  , 

c.  E.  M.  F.  of  Motor 170  | 

Cell.  Porous,  of  Voltaic  Cell     7 1 

Cell,  Standard,  Clark's n   | 

Cell,  Voltaic,  Bunsen 75,     76 

Cell,  Voltaic,  Callaud 77 

Cell,  Voltaic,   Comparative 

Constancy  of  E.  M.  F.  of  .86,    87 
Cell,  Voltaic.  Cost  of  Cur- 
rent of  81,     82  j 

Cell,  Voltaic,  Darnell ...  76,     7? 
Cell,  Voltaic,  Daniell,  E.M.F. 

of 78 

Cell,   Voltaic,  Definition  of    65 
Cell,    Voltaic,     Edison-La- 

lande  79  | 

Cell,  Voltaic,   Electrical 

Capability  of 83,     84 

Cell,    Voltaic,    GYavity, 

Daniell 77 

Cell,  Voltaic,  Grove 75 

Cell,  Voltaic,  Maximum 
Economy    of    Operation 

of ^ 84,     85 

Cell,  Voltaic,  Negative  Pole 

of,  Definition  of 69 

Cell,  Voltaic,  Poggendorf's 

73.  74 

Cell,  Voltaic,  Silver  Chlor- 
ide  79,  80 

Cells,  Voltaic,  Grouping  of  85 

c.  G.  s.  Unit  of  Resistance..  18 
Chamber  of    Incandescent 

Lamp,  Blackening  of ....  211 

Charge,  Electric i  to  8 

Circuit,      Aero-Ferric,      of 

Electromagnet 108 

Circuit,  Constant-Current.  59 

Circuit,  Constant-Potential  61 


Circuit  Containing  C.E.M.F., 
Application    of    Ohm's 

Law  to 53.     54 

Circuit,      Distribution      of 
Potential  Difference  in.  51,  52 

Circuit,  Electric 57  to    64 

Circuit,  Electric,    Essential 

Parts  of 57 

Circuit,  Electric,  Prime  Ob- 
ject of 57 

Circuit,  Ferric-Magnetic  . .   100 
Circuit,  Magnetic,   Aero- 
Ferric  100 

Circuit,  Magnetic,  Varieties 

of TOO 

Circuit,  Metallic 28 

Circuit,  Multiple 60,     61 

Circuit,  Multiple-Series,  De- 
finition of 62,     63 

Circuit,  Non-Ferric-Mag- 
netic     100 

Circuit,   Series,    Definition 

of 58,     59 

Circuits,  Electric,  Varieties 

of,  58 

Clark's  Standard  Cell 1 1 

Classification  of  Electrical 

Effects 4 

Clutch  for  Arc  Lamp  Rod.   225 

Coils,  Resistance 31,     32 

Commercial    Efficiency    of 

Dynamo 137,  138 

Commutation,  Diameter  of,  150 
Commutator    of    Gramme- 
Ring  Armature 134 

Complex-Harmonic  E.  M.  F.  261 
Compound  Alternators  . . .  263 
Compound-Wound  Genera- 
tor, Diagram  of 155 


ItfDEX. 


285 


Compound- Wound  Genera- 
tors, Parallel  Connection 
of 159 

Conductance,  Definition  of     23 

Conductance,  Total  Value  of     23 

Conducting  Loop,  Genera- 
tion of  E.  M.  F.  in,  by  Rota- 
tation  in  Magnetic 
Field 126,  127 

Conductivity  and  Resistiv- 
ity, Relations  Between. . .  25 

Conductivity,  Matthiessen's 
Standard  of 34,  35 

Conductor,  Circumstances 
Affecting  Resistance  of ..  17 

Conductor,  Drop  in 50 

Conductor,  Effect  of  Slight 
Impurities  on  Resistivity 
of 20 

Conductor,  Effect  of  Tem- 
perature on  Resistivity  of  20 

Conductor,  Stranded,  De- 
finition of 37 

Conductor ,  Temperature 
Coefficient  of  Resistivity  21 

Conductor,  Thermal  Activ- 
ity of  Current  in 193 

Conductors,  Bare  Elec- 
trical, Time  Required  for 
Full  Elevation  of  Tem- 
perature in 198 

Conductors,  Electric,  Tem- 
perature Elevation  of . . 

197,  198,  199 

Conductors,  Insulated  or 
Covered,  Time  Required 
for  Full  elevation  of  Tem- 
perature in 198 

Connections  of  Resistance 

Frame 30 


Constant-Current  Circuit . .     59 

Constant-Potential  Circuit.     61 

Constant- Potential  Genera- 
tors, Sparkless  Commuta- 
tion of  154 

Constant- Potential  Incan- 
descent Lamp  Circuits. . 

228,  229 

Contacts,  Resistance  of  35,     36 

Continuous  Current,  Defini 
tion  of 14 

Continuous-Current  Elec- 
tric Motors 169  to  192 

Continuous-Current  Gene- 
rators, Classification  of . .  156 

Continuous-Current  Mo- 
tors, Classification  of  Con- 
trolling Factors  in  Opera- 
tion of  171,  172 

Continuous  E.  M.  F.,  Defini- 
tion of 14 

Copper  Wire,  Eddy-Current 
Losses  in 139,  140 

Copper  Wires,  Safe  Carry- 
ing Capacity  of 199 

Copper  Wires,  Tables  of 
Resistance  of . .  35 

Cored  Carbons  for  Arc 
Lights 220 

Cost,  Minimum,  Requisite 
Conditions  for  Obtaining, 
in  Battery  Installations. .  8  > 

Coulomb,  Definition  of  ....     42 

Coulomb  Meter 43 

Counter  Elect ro motive 
Force  of  Voltaic  Arc  ....  218 

Couple,  Voltaic 66 

Crater,  Positive,  of  Voltaic 
Arc,  Formation  of, 219 


286 


INDEX. 


Crater,  Positive,  of  Voltaic 
Arc,  Temperature  of 219 

Critical  Output  of  Dynamo- 
Electric  Machine 145 

Current,  Alternating,  De- 
finition of  14,  233 

Current ,  Alternating  or 
Periodic.  Peaked  Type  .  235 

Current,  Continuous,  De- 
finition of 14,  44 

Current,  Electric 41  to    48 

Current,  Electric,  Electro- 
lytic Effect  of 42,  43 

Current,  Electric,  Heating 
Effect  of 3 

Current,  Electric,  Moment- 
ary    3 

Current,  Electric,  Rate  of 
Flow  43 

Current,  Fluctuating  or 
Pulsatory 233 

Current,  Periodic  or  Alter- 
nating, Flat-Topped 
Type  of 235 

Current,  Periodic  or  Alter- 
nating, Sinusoidal -Type 
of 235 

Current,  Pulsatory,  Defini- 
tion of 44 

Current,  Simple-Periodic  . .  236 

Current,  Thermal  Activity 
of  in  Conductor 193 

Currents,  Alternating,  De- 
finition of 45 

Currents,  Diphase 267 

Currents,  Electric, Varieties 
of 44 

Currents,  Monocyclic 267 

Currents,  Multiphase 267 


Currents,  Table  of  Varying 
Commercial  Strengths  of  45 

Currents,  Triphase 267 

Curves  of  Reluctivity  99 

Cut-out,  Automatic,  for  Arc 

Lamps  226,  227 

Cycle,  Definition  of 238 

Cycle,  Hysteretic 142,  143 

Daniell  Voltaic  Cell 76,     77 

Depolarizer,  Solid,  for  Vol- 
taic Cell 69 

Depolarizer  for  Voltaic  Cell    68 
Diameter  of  Commutation .   150 

Difference  of  Potential 16 

Diphase  Currents 267 

Diphase  E.  M.  F.  's 267 

Direction  of  Magnetic  Flux, 

Convention  as  to 88,     89 

Discharge,  Electric,  Mo- 
mentary   • 2 

Dissymmetrical  E.  M.  F.  ,  De- 
finition of 14 

Direction  of  Magnetization, 
Dependence  of,  on  Direc- 
tion of  Current  114 

Divided  Circuit, Application 

of  Ohm's  Law  to 52 

Double-Carbon  Arc  Lamps  224 

Drop  in  Conductor 50 

Drum- Wound  Armature. . .   130 
Dynamic  Force,  Definition 

of 169,  170 

Dynamo  and  Motor,  Co- 
existence of  Electrodyna- 
mic  and  Electromotive 

Force  in 169 

Dynamo  and  Motor,  Re- 
versibility of  169 

Dynamo-Electric  Induction  121 


INDEX. 


287 


Dynamo-Electric  Induction 
in  Conductor,  three  cases 
of 122,  123 

Dynamo-Electric  Machine, 
Building  up  of 158,  1 59 

Dynamo- Electric  Machine, 
Classification  of  Losses  in  138 

Dynamo-Electric  Machine, 
Commercial  Efficiency  of 

137,  138 

Dynamo-Electric  Machine, 
Critical  Output  of 145 

Dynamo-Electric  Machine, 
Electrical  Capability  of..  135 

Dynamo-Electric  Machine, 
Essential  Parts  of  .  129 

Dynamo-Electric  Machine, 
Lead  of  Brushes  in ......  1 50 

Dynamo-Fvlectric  Machine, 
Limitation  of  Output  by 
Excessive  Drop  of  Arma- 
ture  145,  146 

Dynamo-Electric  Machine, 
Limitation  of  Output  by 
Excessive  Heating. . .  146,  147 

Dynamo-Electric  Machine, 
Limitation  of  Output  by 
Excessive  Sparking  at 
the  Brushes  148,  149 

Dynamo-Electric  Machine, 
Relation  Existing  be- 
tween Output  and  Inter- 
nal Resistance 134,  135 

Dynamo,  Electrical  Losses 
of 138 

Dynamo,  Hysteretic  Losses 
in 140 

Dynamo,  Magnetic  Losses 
of 138 


Dynamo,  Mechanical  Losses 

of 138 

Dynamo,  Output  of 134 

Dynamo,  The 129  to  152 

Dynamos,  Series  Connec- 
tion of 59,  60 

E.  M.  F.,  Alternating,  Defini- 
tion of 14,  233 

E.  M.  F.,  Complex-Harmo- 
nic   261 

E.  M.  F.,  Continuous,  Defi- 
nition of 14 

E.  M   F.,  Counter,  of  Motor.   170 

E.  M.  F.,  Definition  of  Direc- 
tion of  9 

E.  M.  F.,  Dissymetrical,  De- 
finition of 14 

E.  M.  F.,  Fluctuating,  De- 
finition of 14 

E.  M.  F.  in  Armature  Con- 
ductors, Calculation  of 
Value  of 131 

E  M.  F.  Induced  in  Rotating 
Conducting  Loop,  Direc- 
tion of 132 

E.  M.  F.  of  Armature  Con- 
ductor,  Rule  for  Calculat- 
ing    134 

E.  M.  F.  or  Current,  Periodic 
or  Alternating,  Rectan- 
gular Type  of 234 

E.  M.  F.  or  Current,  Periodic 
or  Alternating,  Zigzag 
Type  of 234 

E.  M.  F.  or  Current,  Periodic 
Peaked  Type  of 235 

E.  M.  F.,  Periodic,  or  Alter- 
nating Current,  Flat- 
Topped-Type  of 235 


288 


INDEX. 


E.  M.  F.,  Periodic,  or  Alter- 
nating Current,  Sinus- 
oidal Type  of 235 

E.  M.  F.,   Production  of,  by 

Mutual  Induction 128 

E.  M.  F.,  Pulsating,    Defini- 
tion of . .     14 

E.  M.  F.,  Simple-Harmonic.  236 
E.  M.  F.,  Simple-Periodic...  236  | 
E.  M.  F.,  Steady,   Definition 

of 14 

E.  M.  F.,  Symmetrical,  De- 
finition of  14 

E.  M.  F.,  Triphase,  Diagram 

of 270 

E.  M.  F.,  Unit  of ii 

E.  M.  F.'S,  Conjoined 9,     10 

E.  M.  F.'S,  Diphase 267 

E.  M.  F.'S,  Opposed  Action 

of 9,     10 

E.  M.  F.'S  or  Currents,  Fluc- 
tuating or  Alternating, 
Graphic  Representation 

of 234 

E.  M.  F.'S  or  Currents,  Sinu- 
soidal, Harmonics  of 260 

E.  M.  F.'S,  Triphase 269,  270 

Earth's  Crust,  Average  Re- 

sistivity  of 27 

Earth's  Flux,  Production  of 
E.  M.  F.  in  Coil  by  Rotation 

in 127 

Eddy-Ctirrent  Losses 139 

Eddy-Current  Losses  in 

Copper  Wire 139,  140 

Edison-Lalande  Voltaic 

Cell 79 

Effect,  Skin,  Definition  of..  198 
Effective  or  Joint  Resistance    23 


Efficiencies,  Varying,  Com- 
mercial,  for  Incandescent 
Lamps 207,  208 

Efficiency  and  Size  of  Trans 
former,  Effect  of  Fre- 
quency of  Alternation  on  279 

Efficiency,  Commercial,  of 
Dynamo 137,  138 

Efficiency,  Electrical,  of 
Dynamo 137 

Efficiency  of  Alternating- 
Current  Transformers. . .  278 

Efficiency  of  Distribution 
of  Electrical  Heating. ...  195 

Efficiency  of  Incandescent 
Lamp 206 

Efficiency  of  Machine 7,       8 

Electric  Charge i  to       8 

Electric  Current 41  to    48 

Electric  Currents,  Magnetic 
Effects  of 4 

Electric  Currents,  Varieties 
of 44 

Electric  Motor,  Advantages 
Possessed  by 188,  189 

Electric  Motor,  Conditions 
for  Constant  Torque  and 
Variable  Speed 178,  179 

Electric  Motor,  Conditions 
for,  Variable  Torque  and 
Constant  Speed 181 

Electric  Motor,  Conditions 
for,  Variable  Torque  and 
Variable  Speed 182 

Electric  Sources,  Series- 
Connected 59,  60 

Electrical  Capability  of 
Dynamo-Electric  M  a  - 
chine 135, 


Electrical  Effects 


136 
l  to       8 


INDEX. 


289 


Electrical  Effects,  Classifi- 
cation of 4 

Electric  Heater,  Practical 
Efficiency  of 196 

Electric  Losses  of  Dy- 
namo   138 

Electricity,  Unit  Quantity 
of 42 

Electricity,  Unit  Rate  of 
Flow  of 43 

Electrification ,  Effect  of 
Surface-Dissimilarity  on  2 

Electrode,  Negative,  of 
Voltaic  Cell 69 

Electrode,  Positive,  of  Vol- 
taic Cell 69 

Electrodes  for  Arc  Lamps, 
Length  of 223 

Electrodynamic  Attractions 
and  Repulsions 167,  168 

Electrodynamic  Force 162 

Electrodynamic  Force,  Cir- 
cumstances Affecting 
Value  of 162,  163 

Electrodynamic  Force, 
Source  of  Work  Done  by  165 

Electrodynamics 161  to  168 

Electrodynamics,  Defini- 
tion of 161 

Electrolysis,  Definition  of..      4 

Electrolyte,  Definition  of . .     65 

Electrolytic  Effect  of  Elec- 
trical Current 42,  43 

Electromagnet,  Aero-Ferric 
Circuit  of 108 

Electromagnet,  Definition 
of 113 

Electromagnets 1 13  to  120 

Electromotive  Force ...  9  to    16 


Electromotive  Force,  Dy- 
namo-Electric Induction 
of,  in  Conducting  Loop 

124,   125 

Electromotive  Force,  Gen 
eration  of,  in  Conducting 

Loops 124 

Electromotive  Force, Origin 

of 16 

Electroplated  Arc-Light 

Carbons 220 

Element,  Voltaic,  Defini- 
tion of 66 

Energy   Constancy  of 6 

Energy,  Definition  of 5 

Energy,  Expenditure  of . .         5 

Energy,  Wasted 7 

Ether,  Universal 2 

Exhaustion  of  Lamp  Cham- 
ber of  Incandescent  Lamp  203 
Exploring        Magnetic 
Needle 92 

Feeding  Mechanism  for 
Arc  Lamps 222 

Ferric-Magnetic  Circuit —   100 

Field  Magnets,  Function  of  129 

Filament  of  Incandescent 
Lamp,  Cause  of  Decrease 
in  Diameter  of 209 

Filament  of  Incandescent 
Lamp,  Cross  Sectional 
Area  of 205 

Filament  of  Incandescent 
Lamp,  Decrease  of  Di- 
ameter and  Efficiency  of.  210 

Filament  of  Incandescent 
Lamp,  Flashing  Process 
for 203 


290 


INDEX. 


Filament  of  Incandescent 
Lamp,  Manufacture  of  . . 

201,  202 

Filament  of  Incandescent 
Lamp,  Materials  Employ- 
ed in 202 

Filaments  for  Incandescent 
Lamp,  Mounting  of 203 

Flashing  Process  for  Fila- 
ment of  Incandescent 
Lamp 203 

Flat-Topped  Type  of  Peri- 
odic E.M.F.  or  Alternating 
Current 235 

Fleming's  Hand  Rule 123 

Fleming's  Hand  Rule  for 
Motors 163 

Fluctuating  Alternating 
E.  M.  F.'S,  Graphic  Repre- 
sentation of 234 

Fluctuating  E.  M.  F.,  Defini- 
tion of...  14 

Fluctuating  or  Pulsatory 
Current 233 

Flux-Paths,  Magnetic,  De- 
finition of 89 

Flux,  Prime 113 

Focusing  Arc  Lamps 223 

Following  or  Trailing  Edge 
of  Motor  Armature 186 

Force,  Dynamic,  Definition 
of 169,  179 

Force,  Electrodynamic 162 

Force,  Electrodynamic, Cir- 
cumstances Affecting 
Value  of 162,  163 

Franklin's  Kite 3 

Frequency,  Definition  of..  238 

Frequency,  Effect  of,  on 
Skin  Effect 256 


Frequency  of  Alternation, 
Effect  of,  on  Size  and  Effi- 
ciency of  Transformer. . .  279 

Frequency  of  Incandescent 
Lighting  Alternators 265 

Galvanometers 46 

Galvanometer,      Thomson 

Mirror. ...   47 

Generator,   Compound- 
Wound,  Diagram  of    ...  155 
Generator,    Series-Wound, 

Diagram  of 154 

Generator,    Shunt- Wound, 

Diagram  of 155 

Generators  and  Motors, 
Relative  Direction  of 

Rotation  in 187,  188 

Generators,     Classification 

of  Continuous-Current ...  156 
Generators,  Dynamo  Elec- 
tric, Classification  of 156 

Gilbert,  Definition  of . .  .92,     95 

Gradient,  Hydraulic 15 

Gradient,  Potential 15 

Gramme-Calorie,  Definition 

of 193 

Gramme-Ring     Armature, 

Commutator 134 

Gramme  Ring  Armature 
Dynamo- Electric  Induc- 
tion of  E.  M.  F.  in 133 

Graphite,  Formation  of,  in 

Voltaic  Arc ...   219 

Gravity  Daniell  Voltaic  Cell     7 7 

Grenet  Cell,  E.  M.  F.  of 73 

Grenet  Voltaic  Cell 73,     74 

Ground  Plates,  Telegraph- 
ic, Definition  of 27 


INDEX. 


291 


Ground-Return  Circuit,  De- 
finition of 28 

Grove  Voltaic  Cell 75 

Hand  Rule,  Fleming's 123 

Hard  Iron,  Action  of  Mag- 
netic Flux  on 113 

Heat,  Commercial  Applica- 
tions of  Electrically  Gene- 
rated   194 

Heater,    Electric,    General 

Construction  of 195 

Heater,  Electric,  Practical 

Efficiency  of 196 

Heating  Effect  of  Alternat- 
ing Current 239 

Heating  Effect  of  Electric 

Current 3 

Heating,  Electric. ...  193  to  200 
Heating,  Electric,  Efficien- 
cy of  Distribution  of 195 

Hefner- Alteneck  Lamp . . .  206 
Horizontal     Candle-Power 

for  Arc  Lamps 222 

Horse -Power,  Definition  of    12 
Human  Body,  Electric  Re- 
sistance of 39 

Hydraulic  Gradient 15 

Hysteresis,  Magnetic 140 

Hysteretic  Cycle. ......  142,   143 

Hystere tic  Diagrams.  .142,  143 
Hysteretic  Loss  in  Dynamo  140 

Illumination,  Definition  of  209 
Illumination ,    Proposed 

Unit  of  209 

Impedance,  Definition  of..  244 
Impedance  Factor  of  Alter- 
nating-Current Circuit..  255 


Impedance  of  Alternating- 
Current  Circuit,  Quanti- 
ties Entering  into  Value 
of 244 

Incandescent  Arc  Light 
Circuits 228,  229 

Incandescent  Electric 
Lamp,  Mounting  of  Fila- 
ment in 203 

Incandescent  Lamp,  Ac- 
tivity Absorbed  by 205 

Incandescent  Lamp  Bases, 
Varieties  of 204,  205 

Incandescent  Lamp,  De- 
crease of  Diameter  of 
Filament  Attending  Use  210 

Incandescent  Lamp,  Defini- 
tion of 201 

Incandescent  Lamp,  Effect 
of  Variation  in  Diameter 
of  Filament  on  Efficiency 
of v..  210 

Incandescent  Lamp,  Exces- 
sive Non-luminous  radia- 
tion of 197 

Incandescent  Lamp  Fila- 
ment, Materials  Employ, 
ed  in  Manufacture  of 202 

Incandescent  Lamp,  Incor- 
rect Statement  of  Effici- 
ency of 206 

Incandescent  Lamp,  Lead- 
ing-in  Wires  of 202 

Incandescent  Lamp,  Manu- 
facture of  Filament  for . . 

201,    202 

Incandescent  Lamp,  Steps 
in  Manufacture  of 201 

Incandescent  Lamp, Single- 
Pole  Switch 213,  214 


292 


tNDEX. 


Incandescent  Lamp  Switch- 
es    213 

Incandescent  Lamps, 
Candle-Powers  of — 207,  208 

Incandescent  Lighting 

201  to  216 

Incandescent  Lighting,  Al- 
ternating-Current Cir- 
cuits for 212 

I  ncandescent  Lighting , 
Series  Circuits  for 212 

Incandescent  Lighting , 
Systems  of  Distribution 
for 212 

Induced  Electromotive 
Force 121  to  128 

Inductance  of  Alternating- 
Current  Circuit 244 

Inductance-Reactance  .245,  246 

Induction,  Dynamo-Elec- 
tric   121 

Induction,  Magneto-Elec- 
tric  121 

Induction,  Mutual 122 

Induction,  Self- 121 

Induction,  Self-,  Production 
of  E.  M.  F.  by 127,  128 

Inductor  Alternators..  257,  258 

International  Ampere,  De- 
finition of 43 

International  Unit  of  Activ- 
ity or  Power,  Definition  of  12 

International  Unit  of  Work, 
Definition  of u,  12 

International  Ohm 17,     18 

Insulation,  Apparent,  of 
Telegraphic  Line 29 

Insulation, Average  Appar- 
ent per  mile  of  . .  29 

Insulators,  Definition  of . . .     19 


Insulators,  Effect  of  Tem- 
perature on  Resistivity  of  22 

Insulators,  Varieties  of 28 

Intake  of  Machine,  Defini- 
tion of 7 

Iron,  Action  of  Magnetic 
Flux  on 113 

Iron,  Eddy-Current  Losses 

in 139 

Iron,  Reluctivity  of 104 

Jablochkoff  Candles 222 

Joint  Admittance.. 249,  250,  251 

Joint  Resistance 19 

Joule,  Definition  qf u,     12 

Joule  per  Second,  Defini- 
tion of. . .  12 

Kilovolt,  Definition  of u 

Kilowatt,  Definition  of ....     12 
Kite,  Franklin's 3 

Lamp  Chamber  of  Incan- 
descent Lamp,  Exhaus- 
tion of 203 

Lamp,  Incandescent,  De- 
finition of 201 

Lamp,  Life  of 211 

Lamp  Rod,  Clutch  for 225 

Lead  of  Commutator 
Brushes,  Effect  of,  on 
Sparking 150,  151 

Leading  Polar  Edge  of 
Motor  Armature 186 

Leading-in  Wires  of  Incan- 
descent Lamp 202 

Leading  in  Wires  of  Incan- 
descent Lamp,  Method 
Employed  for  Connect- 
ing with  Filament. 202 


INDEX. 


293 


Leclanche  Voltaic  Cell 78 

Leclanche      Voltaic      Cell, 

E.  M.  F.  Of 78 

Lesser  Calorie, Definition  of  193 
Life  of  Incandescent  Lamp  211 
Lifting  Magnets  for  Arc 

Lamps,  Shunt  and  Series 

Winding  for 227 

Line,  Average  Apparent 

Insulation  per  mile  of  ...  29 
Lines  of  Magnetic  Force, 

Definition  of. 89 

Losses,  Eddy-Current 139 

Losses  of  Dynamo 138 

M.  M.  F 92 

M.  M.  F.'S,  Joint,  or  Op- 
posed, Action  of 95 

M.  M.  F.,  Relation  of,  to  Am- 
pere Turns 94 

Machine,  Definition  of  Out- 
put of 7 

Machine,  Dynamo-Electric, 
Essential  Parts  of 129 

Machine,  Efficiency  of ..  .7,       8 

Machine,  Intake  of,  Defini- 
tion of 7 

Machines,  Dynamo-Elec- 
tric, Armatures  of 130 

Magnet,  Polar  Surfaces  of  115 

Magnets,  Attractive 115 

Magnets,  Portative 114 

Magnetic  and  Electric  Re- 
sistances, Differences  be- 
tween  97,  98 

Magnetic  Circuits,  Compu- 
tation of  Reluctance  in . . 

100,  101,  102,  103 

Magnetic  Effects  of  Elec- 
tric Currents 4 


Magnetic  Figures,  Methods 
of  Obtaining, 91,  92 

Magnetic  Flux 105  to  112 

Magnetic  Flux,  Action  of, 
on  Hard  Iron 113 

Magnetic  Flux,  Action  of, 
on  Soft  Iron 113 

Magnetic  Flux,  Calculation 
of 106,  107 

Magnetic  Flux,  Convention 
as  to  Direction  of 88,  89 

Magnetic  Flux-Paths,  De- 
finition of 89 

Magnetic  Flux,  Properties 
of 89 

Magnetic  Force,  Definition 
of  Lines  of 89 

Magnetic  Hysteresis 140 

Magnetic  Losses  of  Dynamo  138 

Magnetic  Needle,  Explor- 
ing   92 

Magnetic  Reluctance.  97  to  104 

Magnetic  Reluctance,  De- 
finition of 97 

Magnetism,  Permanent  ...   114 

Magnetism,  Temporary  ...   114 

Magnetization,  Influence  of 
Direction  of  Current  on 
Direction  of 1 14 

Magneto-Dynamics,  Defini- 
tion of 161 

Magneto-Electric  Induc- 
tion   121 

Magnetomotive  Force.. 8 7  to  96 

Magnetomotive  Force,  De- 
finition of 92 

Magnetomotive  Force,  Di- 
rection of 95 

Magnetomotive  Force,  Per- 
manent   92 


INDEX. 


Magnetomotive     Force, 

Structural 114 

Magnetomotive      Force, 

Transient 92,  93 

Magnetomotive      Force, 

Unit  of 92 

Matthiessen's   Standard  of 

Conductivity 34,  35 

Maximum  Candle-Power  of 

Arc  Lamps 222 

Mean    Spherical     Candle - 

Power  of  Arc  Lamp 220 

Mechanical  Losses  of  Dy- 
namo   138 

Megawatt,  Definition  of . . .  12 

Megohm,  Definition  of 20 

Megohm-Mile 29 

Metallic  Circuit 28 

Method  of  Reversing  Elec- 
tric Motors 189 

Mho,  Definition  of 25 

Microhm,  Definition  of. . .  20 

Microvolt,  Definition  of  ...  1 1 

Millivolt,  Definition  of 1 1 

Mine  Hoist,  Electric 178 

Mirror  Galvanometer, 
Thomson    Tripod    Form 

of 46,  47 

Molten  Platinum  Lamp  or 

Violle 206 

Momentary    Electric    Cur- 
rent     3 

Momentary    Electric     Dis- 
charge   3 

Monocyclic  Currents 267 

Monocyclic     System,     De- 
scription of 271,  272 

Motion,  Simple-Harmonic.  236 

Motion,  Simple-Periodic. ..  236 


Motor  and  Generator,  Dis- 
tinctive Features  of  Dif- 
ferences between 170 

Motor- Armature ,  Defini- 
tion of  Leading  Polar 
Edge  of 186 

Motor-Armature,  Follow- 
ing or  Trailing  Polar 
Edge  of 186 

Motor- Armature,  Smooth 
Core,  Use  of 185 

Motor-Armature,  Toothed 
Core 185 

Motor,  c.  E.  M.  F.,  of    ....   170 

Motor,  Electric,  Conditions 
for  Constant  Torque  and 
Variable  Speed  in. . .  178,  179 

Motor,  Electric,  Method  of 
Reversing 189 

Motor,  Electric,  Variable 
Torque  and  Variable 
Speed,  Conditions  for...  182 

Motor,  Torque  in  Armature 
of,  Definition  of 166 

Motors  and  Generators, 
Relative  Direction  of 
Rotation  in. 187,  188 

Motors,  Fleming's  Hand 
Rule  for. . .  163 

Motors,  Single-Reduction. .   190 

Motors,  Starting  Rheostat 
for 190 

Multiphase  Currents 267 

Multiple  Circuit,  Definition 
of 60,  61 

Multiple  Circuit,  Resistance 
of 61 

Multiple- Series  Circuit,  De- 
finition of 62,  63 

Municipal-Series  Circuit. . .     58 


INDEX, 


295 


Mutual  Induction 122 

Mutual  Induction,  Produc- 
tion of  E.  M.  F.,  by ,  128 

Negative  Lead,  Definition 
of 62 

Negative  Plate  of  Voltaic 
Cell 70 

Negative  Pole  of  Electric 
Source,  Definition  of ....  13 

Negative  Pole  of  Voltaic 
Cell,  Definition  of 69 

Negative  Resistivity,  Tem- 
perature Coefficient  of .  22 

Non-Conductors,  Definition 
of 19 

Non-Ferric  Magnetic  Cir- 
cuit  100 

Non-luminous  Radiation, 
Excessive  Amount  of,  in 
Incandescent  Lamp. ...  197 

Occluded  Gases,  Process  for 

Exhaustion  of 204 

Ohm,  International 17,     18 

Ohm,  Multiples  and  Sub- 
Multiples  of 19 

Ohm's  Law 49  to     56 

Ohm's  Law,  Application  of, 
to  Circuit  Containing 

c.  E.  M.  F 53,     54 

Ohm's  Law,  Application  of, 

to  Divided  Circuit 52 

Ohm's  Law,  Application  of , 

to  Shunt  Circuit 52,     53 

Ohm's  Law,  Formula  of. . .     50 
Ohm's  Law,  Limitation  of    54 
Output,  Critical,  of  Dyna- 
mo-Electric Machine     . .   145 
Output,  Electrical 196 


Output  of  Alternating-Cur- 
rent Transformers,  Limi- 
tations of  . .  275 ,  276 

Output  of  Dynamo 134 

Output  of  Dynamo,  Limita- 
tion of  ,by  Excessive  Drop 
in  Armature 145,  146 

Output  of  Dynamo,  Limita- 
tion of,  by  Excessive 
Heating 146,  147 

Output  of  Dynamo,  Limita- 
tion of,  by  Dangerous 
Sparking  at  Brushes.  148,  149 

Output  of  Machine,  Defini- 
tion of 7 

Overload  of  Safety  Fuse  . .  200 

Parallel  Connection  of  Elec- 
tric Sources  ... 61,  62 

Parallel  Connection  of 
Shunt-Wound  Genera- 
tors   157,  158 

Parallel-Series  Alternator..  262 

Peaked  Type  of  Periodic 
E.  M.  F.  or  Alternating 
Current 235 

Period,  Definition  of 238 

Periodic  Alternating  E.  M.  F. 
or  Current,  Flat-Topped 
Type  of 235 

Periodic  Alternating  E.  M.  F. 
or  Current,  Peak -Topped 
Type  of 235 

Periodic  Alternating  E.M.  F. 
or  Current,  Sinusoidal 
Type  of 235 

Permanent  Magnetism ....  114 

Permanent  Magnetomotive 
Force 92 

Phase,  Definition  of 238 


INDEX. 


Poggendorff's  Voltaic  Cell. 

73,     74 

Polar  Surfaces  of  Magnet..  115 
Polarization  of  Voltaic  Cell  63 
Polarization  of  Voltaic  Cell, 

Effect  of  Resistance  of . .     71 
Polarization  of  Voltaic  Cell, 

How  Avoided 68 

Pole,   Positive,  of  Voltaic 

Cell 69 

Poles  of    Electric  Source, 

Definition  of 13 

Porous  Cell,  of  Voltaic  Cell    71 

Portative  Magnets 115 

Positive  Carbon  of  Voltaic 
Arc,  Rate  of  Consump- 
tion of 219 

Positive    Lead,    Definition 

of 62 

Positive  Plate   of    Voltaic 

Cell 70 

Positive    Pole  of    Electric 

Source,  Definition  of ....     13 
Potential  Difference  in  Cir- 
cuit, Distribution  of .  .51,     52 
Potential,  Difference  of. ...     16 

Potential  Gradient 15 

Power,  Factor  of  Alternat- 
ing-Current Circuit 253 

Power,  Factor  of  Alternat- 
ing-Current Transform- 
ers  278,  279 

Prime  Flux 113 

Prime  Magnetomotive 

Force 113 

Production  of  E.  M.  F.  in  Coil 
by  Rotation  in  Earth's 

Flux 127 

Pulsating  E.  M.  p.,  Defini- 
tion of 14 


Pulsatory  Current,  Defini- 
tion of 44 

Pulsatory  or  Fluctuating 
Current 233 

Quegohm,  Denfinition  of  . .     20 

Radiation,   Physiologically 

Effective,  of  Arc  Lamp. .  221 
Reactance,  Definition  of. . .  245 
Reactance  Factor  of  Alter- 
nating Current  Circuit  . .   255 

Reaction  of  Armature 118 

Regulation  of  Dynamo  . . . 

153  to  160 

Reluctance,  Specific  Mag- 
netic  ; 99 

Reluctivity,  Curves  of 99 

Reluctivity  of  Iron 104 

Reluctivity,   Variation    of, 

with  Flux  Density 99 

Resistance,  c.  G.  s.  Unit  of     18 

Resistance  Coils 31,     32 

Resistance,     Effective     or 

Joint 23 

Resistance,  Electric.  .17  to    48 
Resistance  Frame,  Connec- 
tions of 30 

Resistance,  Joint 19 

Resistance  of  Commercial 
Electric  Apparatu  s, 

Tables  of 37,  38,     39 

Resistance    of    Conductor, 

Circumstances  Affecting  17 
Resistance  of  Contacts.. 35,  36 
Resistance  of  Copper  Wires, 

Table  of 35 

Resistance  of  Human  Body    39 
Resistance  of  Multiple  Cir- 
cuit. .      61 


INDEX. 


297 


Resistance  of  Series  Cir- 
cuits   25 

Resistance  of  Series  Cir- 
cuit   59 

Resistance,  Specific,  Defini- 
tion of 20 

Resistance,  Unit  of 17,     18 

Resistance,  Variable. . .  .29,     30 
Resistivity,     Average,     of 

Earth's  Crust 27 

Resistivity,  Definition  of..     20 
Reluctivity,  Definition  of . .     98 
Resistivity  of  Conductors, 
Effect    of  Slight  Impur- 
ities on 20 

Resistivity    of    Conductor, 

Effect  of  Temperature  on     20 
Resistivity    of    Insulators, 
Effect  of  Temperature  on     22 

Resistivity,  Table 21 

Resistivity,      Temperature 

Coefficient  of  Conductor.     21 
Reversibility    of    Dynamo 

and  Motor 169 

Return     Circuit,     Ground, 

Definition  of 28 

Rheostat,  Carbon 36 

Rheostat, Starting, for  Elec- 
tric Motors 190 

Ring- Wound  Armature. ...   130 
Rotation,    Relative    Direc- 
tion of,  in  Generators  and 

Motors 187,  188 

Rule  for  Determining  Di- 
rection of  E.  M.  F.  Induced 
in  Rotating  Loop 132 

Safe  Carrying  Capacity  of 
Copper  Wires  199 


Safety  Device  of  Series- 
Connected  Circuit 59 

Safety  Fuse,  Overload  of. .   200 

Search  Lights,  Use  of 
Large  Arcs  in  218 

Self-Excited  Alternators. . .   263 

Self-induction    121 

Self -Induction,  Production 
of  E.  M.  F.  by 127,  128 

Self  Regulating  Character 
of  Alternating-Current 
Transformer  274 

Semi-Period,  Definition  of  238 

Separately-Excited  Alter- 
nators    263 

Series  Arc  Circuits,  Switch- 
board for 230 

Series  Circuit,  Definition 
of  58,  59 

Series  Circuit,  Municipal, 
for  Incandescent  Lamps  59 

Series  Circuit,  Resistance 
of 59 

Series  Circuits  for  Incan- 
descent Lighting 212 

Series  Circuits,  Resistance 
of 25 

Series-Connected  Circuit, 
Safety  Device  of 59 

Series-Connected  Electric 
Sources 59,  60 

Series-Connection  of  Alter- 
nators   241 

Series- Connection  of  Arc 
Lamps.  227,  228 

Series- Wound  Alternators .  262 

Series -Wound  Generator, 
Diagram  of 1 54 

Shunt  Circuit,  Application 
of  Ohm's  Law  to 52,  53 


TJKI7BRSITY 


298 


INDEX. 


Shunt  Magnet,  Use  of,  in 
Arc  Lamp 226 

Shunt  Wound  Generator, 
Diagram  of 155 

Shunt-Wound  Generators, 
Parallel  Connection  of,  1 5  7,  158 

Silver  Chloride  Voltaic 
Cell 79,  80 

Silver  Chloride  Voltaic 
Cell,  E.  M.  F.  of So 

Simple-Harmonic  E.  M.  F.  . .   236 

Simple-Harmonic  Motion.. 

236,  237 

Simple-Periodic  Current. . .  236 

Simple-Periodic  E.  M.  F 236 

Simple-Periodic Motion,237,  236 

Single-Pole  Switch  for  In- 
candescent Light  ,..213,  214 

Sinusoidal  Alternators,Con- 
nection  of,  in  Quadrature 

243,  244  I 

Sinusoidal  Type  of  Peri- 
odic E.  M.  F.  or  Alternat- 
ing Current 235 

Sizes  of  Incandescent 
Lamps 207,  208 

Skin  Effect,  Definition  of . .   198 

Skin  Effect,  Effect  of  Fre- 
quency on 256 

Smooth-Core  Motor  Arma- 
ture    185 

Smooth-Core  Motor  Arma- 
ture, Effect  of  Eddy  Cur- 
rents on 185,  186 

Soft  Iron,  Action  of  Mag- 
netic Flux  on 113 

Source,  Definition  of  Posi- 
tive Pole  of 13 

Source,  Electric,  Definition 
of 13 


Source,  Negative  Pole  of, 
Definition  of 13 

Sources  Electric,  Classifi- 
cation of  13 

Sources,  Electric,  Defini- 
tion of  Poles  of, 13 

Sources,  Electric,  Parallel 
connection  of 61,  62 

Sparking  of  Commutator, 
Effect  Lead  of  Brushes  on 

150,  151 

Specific  Heat,  Definition  of  193 

Specific  Magnetic  Reluc- 
tance    99 

Specifications  for  Incan- 
descent Lighting 215 

Standard  Candle,  Defini- 
tion of  206 

Star-Triphase  Winding 270 

Steady  E.M.F.,  Definition  of     14 

Standard  Conductor,  Defi- 
nition of 37 

Structural  Magnetomotive 
Force 114 

Surfaces,  Disimilarity  of, 
Effect  of,  on  Electrication  2 

Switchboard  for  Series  Arc 
Circuits 230 

Switches,  Incandescent 
Lighting 213 

Symmetrical  E.  M.  F.,  Defi- 
nition of 14 

System,  Astatic 48 

System,  Monocyclic,  Des- 
cription of .  ..  271,  272 

System,  Three- Wire 63 

Table  of  Resistance  of  Cop- 
per Wires 35 

Table  of  Resistivity 21 


INDEX. 


299 


Table  of  Varying  Strengths 
of  Commercial  Currents.  45 

Tables  of  Resistance  of 
Commercial  Apparatus. . 

37,  38,     39 

Telegraphic  Ground  Plates, 
Definition  of  27 

Telephone  Transmitter 36 

Temperature  Coefficient  of 
Resistivity  of  Conductor.  2 1 

Temperature  Coefficient  of 
Resistivity,  Negative 22 

Temperature  Effect  on  Re- 
sistivity of  Insulators. ...  22 

Temperature  Elevation  in 
Dynamo  Electric  Machine  148 

Temperature  Elevation, 
Time  Required  for  Maxi- 
mum, in  Insulated  or 
Covered  Conductors 198 

Temporary  Magnetism.  ...   114 

Terminal,  Negative,  of  Vol- 
taic Cell 69 

Terminal,  Positive,  of  Vol- 
taic Cell 69 

Therm 193 

Thomson  Mirror  Galvano- 
meter   47 

Thomson  Mirror  Galvano- 
meter,TripodFormof,46,  47 

Three-Wire  System 62 

Toothed-Core  Motor  Arma- 
ture    185 

Toothed  Dynamo  Armature  152 

Torque  of  Motor  Armature, 
Definition  of 166 

Total  Conductance,  Value 
of 23 

Transformer,  Alternating- 
Current,  Construction  of  273 


Transient    Magnetomotive 

Force 92,     93 

Transmitter,  Telephone. ..  36 
Tregohm,  Definition  of. ...  20 
Triangle-Triphase  Winding  270 

Triphase  Currents 267 

Triphase  E.M  p.,  Diagram  of  270 

Triphase  E.  M.  F.'S 269,  270 

Triphase  Generator,  Star 
Winding  of 270 

Unit  of  Magnetomotive 
Fcrce 92 

Unit  of  Resistance 17,     18 

Unit  of  Work,International, 
Definition  of . n,  12 

UnitQuantity  of  Electricity    42 

Unit  Quantity  of  Heat, 
Names  for 193 

Unit  Rate  of  Flow  of  Elec- 
tricity   43 

Universal  Ether 2 

Variable  Resistances  .  ..29,     30 
Ventilation  of  Dynamo  Ar- 
mature    148 

Violle  or  Molten  Platinum 

Lamp 206 

Volt  or  Unit  of  E.  M.  F.  .    . .     1 1 
Voltaic  Arc,  Counter-Elec- 
tromotive Force  of  ..    ..   218 
Voltaic  Arc,  Definition  of. .   217 
Voltaic  Arc,  Development 
of  Counter-Electromotive 

Force  in 218 

Voltaic  Arc,  Formation  of 

Graphite  in 219 

Voltaic  Cell 65  to     88 

Voltaic  Cell,  Bichromate,  73,     74 


300 


INDEX. 


Voltaic  Cell,  Effect  of  Po- 
larization on  Resistance 
of 71 

Voltaic  Cell,  Fields  of  Use- 
fulness for 87,  88 

Voltaic  Cell,  Grenet ....  73,     74 

Voltaic  Cell,  Phenomena 
Attending  Action  of 67 

Voltaic  Cell,  Polarization 
of 68 

Voltaic  Cell,  Positive  Pole 
of 69 

Voltaic  Cell,  Source  cf  En- 
ergy in 67 

Voltaic  Cells,  Classification 
of 68 

Voltaic  Cells,  Single-Fluid    69 

Voltaic  Couple 66 


Voltaic  Element 66 

Voltmeter,  Definition  of ,  5 1 ,  52 

Water  -  Gramme  -  Degree  - 

Centigrade 193 

Water-Motive  Force,  Orig- 
in of     16 

Watt,  Definition  of 12 

Waves,  Sinusoidal,  of  E.M.F. 

or  Current,  Harmonics  of  260 

Weston  Ammeters         ....  46 
Wheatstone's  Balance... 31, 

32,  33,  34 
Wheatstone's  Bridge. ..  .31, 

32,  33,  34 

Winding,  Star-Triphase...  270 
Winding,Triangle-Triphase  270 

Work,  Defininition  of 5 


ERRATA. 

Page  45,  ^  53,  New  York  City  Lighting.  For  50  read  60  kilo  amperes. 
"    48,  Syllabus.     For  11.16  read  0.1116.  " 
"     52,  ^  59.     For  5,  10,  5  read  50,  100,  50.  ~ 
"     79,  ^  88.     For  0.007  read  0.07.  - 
*'  120,  Syllabus.     For  5.771  X  10~7  read  5.771  (fc8  X  10~7.  - 


DBI7BRSITT 


